Add configuration.removeOnSession()

[robinmacharg]

Goal

To add the {add,remove}OnSession() methods to Bugsnag and BugsnagConfiguration to bring it in line with other notifiers.

Design

Simple. The relevant property is an ObjC array; all necessary checks are performed by the type system and array implementation. Testing ensures that the method performs as expected.

Changeset

Two methods added to Bugsnag and BugsnagConfiguration and tests.

Tests

Quite comprehensive testing of both addition and removal of blocks.

Review

Outstanding Questions

• This pull request is ready for:
  • Initial review of the intended approach, not yet feature complete
  • Structural review of the classes, functions, and properties modified
  • Final review

• The correct target branch has been selected (master for fixes, next for
  features)
• Consistency across platforms for structures or concepts added or modified
• Consistency between the changeset and the goal stated above
• Internal consistency with the rest of the library - is there any overlap between existing interfaces and any which have been added?
• Usage friction - is the proposed change in usage cumbersome or complicated?
• Performance and complexity - are there any cases of unexpected O(n^3) when iterating, recursing, flat mapping, etc?
• Concurrency concerns - if components are accessed asynchronously, what issues will arise
• Thoroughness of added tests and any missing edge cases
• Idiomatic use of the language

[fractalwrench]

Nice comprehensive tests! Left a few comments inline - in particular I think it'd be good to determine how to make a deep-copy of BugsnagConfiguration sooner rather than later.