feat: Consolidate internal logging configuration

[robinmacharg]

Goal

Consolidate internal logging configuration to only use the BSG_LOG_LEVEL preprocessor macro.

Design

The smallest change possible: an import and a variable change. Additional first-pass documentation.

Changeset

BSG_KSLogger.h
BugsnagLogger.h

Tests

Manual testing with the example apps, repeated running of pod install. How to automate testing in an appropriate amount of time was not obvious.

Review

Outstanding Questions

• Platform Documentation could do with being improved (but this can be done as a separate task; raised internally)
• Additional docs (e.g. Carthage, Manual) are probably required for completeness (ditto)
• While this has been tested to a reasonable extent, the #ifdef macros are a little soupy; are there any logging edge cases I've missed?

• 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