(feat)Add callback-free methods to leave breadcrumbs

[robinmacharg]

Goal

Add additional methods to leave complex breadcrumbs without requiring a callback to be provided.

Design

Two top-level Bugsnag methods largely following the "if started..." pattern established previously.

Changeset

Primarily the Bugsnag class and header, along with unit tests in ObjC anbd Swift. Several other minor typographical changes are included but can be ignored.

Tests

It was only required to check whether breadcrumbs were set correctly and so unit testing sufficed. A simple test for Swift exposure was also included.

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]

LGTM pending resolution of two minor review comments left inline