Create structured BugsnagThread class

[fractalwrench]

Goal

Creates a publicly accessible BugsnagThread class which holds fields in properties, rather than an NSDictionary.

This is used as a readonly array property on BugsnagEvent, where the user can alter thread information in callbacks. The changeset relies on #528 which has not yet been merged.

Changeset

• Add properties to BugsnagThread which can be edited by the user. Note that these deviate from the spec, specifically thread.name whose value will always be nil by default

• Updated BugsnagEvent thread generation to occur in constructor rather than in the toJson method

• Updated BugsnagEvent JSON serialization to use BugsnagThread internally

Tests

Added unit tests to verify that threads can be parsed from a dictionary/serialized to JSON. Added additional coverage to ensure that thread information is enhanced with isPC and isLR. Relied on existing unit test coverage to verify the event JSON remains unchanged.

[robinmacharg]

Aside from a couple of nits and querying the necessity of a parameter, a few comments would help my understanding. Other than that, Looks good!

[robinmacharg]

LGTM