Enable BoringSSL for Themis iOS cocoapod

[vixentael]

All below valid for Themis iOS/macOS wrapper and examples.

Should be merged after #223 .

I've tested Themis objcwrapper with BoringSSL iOS pod to make it possible to use Themis with BoringSSL and GRPC (as mentioned in #223).

1. I've updated themis.podspec to contain two subspecs: themis-openssl (default) and themis-boringssl. Now users can specify to use BoringSSL as crypto-engine for Themis in their Podfile:

  pod 'themis/themis-boringssl'

See updated spec.

2. iOS Themis with BoringSSL disables using XTS mode for AES, because it's not included into BoringSSL cocoapod (see Allow to disable reference of AES-256-XTS for Themis iOS #329).

3. iOS Themis tests are running for both themis-openssl and themis-boringssl targets.

4. New podspec is checked to be linted and built for iOS and macOS.

[vixentael]

Do not merge until @sgammon confirms that updated Themis is compatible with GRPC :)

[vixentael]

NB: after publishing podspec do not forget to update all examples to use 0.10.1.

[vixentael]

I've decided to update podspec and merge it to master, because I'll switch to other tasks soon, and will forget the context.

[vixentael]

iOS examples were updated to use 0.10.1 and tested.

Main test project (tests/objcthemis) will be linked to use Themis from master in a separate PR (because podspec should be merged to master first).

[vixentael]

Solves #223