Fix golint warnings

[ilammy]

I've installed the latest golint and walked through the warnings. This should improve out future Go Report, but I'm not sure that we'll get 90%+ score. There are still some warnings that we can't fix without breaking backwards compatibility by renaming constants, structures and methods:

cell/cell.go:135:2: don't use ALL_CAPS in Go names; use CamelCase
cell/cell.go:136:2: don't use ALL_CAPS in Go names; use CamelCase
cell/cell.go:137:2: don't use ALL_CAPS in Go names; use CamelCase
compare/compare.go:67:2: don't use ALL_CAPS in Go names; use CamelCase
compare/compare.go:68:2: don't use ALL_CAPS in Go names; use CamelCase
compare/compare.go:69:2: don't use ALL_CAPS in Go names; use CamelCase
keys/keypair.go:63:2: don't use ALL_CAPS in Go names; use CamelCase
keys/keypair.go:64:2: don't use ALL_CAPS in Go names; use CamelCase
session/session.go:23:2: don't use ALL_CAPS in Go names; use CamelCase
session/session.go:24:2: don't use ALL_CAPS in Go names; use CamelCase
session/session.go:25:2: don't use ALL_CAPS in Go names; use CamelCase
session/session.go:29:6: type name will be used as session.SessionCallbacks by other packages, and that stutters; consider calling this Callbacks
session/session.go:210:26: method GetRemoteId should be GetRemoteID
session/session_test.go:20:27: method GetPublicKeyForId should be GetPublicKeyForID

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• Add documentation comments

golint now requires all exported types, methods, and constants to have documentation comments on them. And the comments should have a particular style, starting with the identifier they describe. I've added some basic comments for all structs and methods.

• Unexport implementation details

Secure Message mode constants should not be exported, they are for internal use. Spell them with a lowercase initial letter instead of uppercase.

• Replace snake_case with camelCase

Go code style requires to use camelCase for variables, not snake_case.

• Spell ID as "ID", not "Id"

golint warns us about incorrect spelling, fix it where possible.

• Drop explicit zero initialization

Values are zero-initialized by default, golint warns about explicit zero initialization. Remove it in Secure Cell code.

[vixentael]

All good except backwards compatibility break for SecureMessage, plz remove it from current PR

[vixentael]

Also: shall update all code in Wiki examples to use new syntax

[ilammy]

The constants in gothemis/message/message.go which are renamed were added only recently in #400. I did not intend them to be exported (and at that time I was not aware of Golang's scoping rules).

Since we did not expose this API in 0.10, I think we're okay with renaming the constants directly right now: the users will not see them in 0.11 so this should not break existing code.

[vixentael]

oh, if they are not exposed to the end users – I'm quite happy then :)

[Lagovas]

lgtm