refactor: rename Bugsnag.addAttribute to addMetadata, add tests

[robinmacharg]

Goal

Simple rename of Bugsnag.addAttribute to Bugsnag.addMetadata to add consistency across notifier platforms.

Design

Simple search and replace. Unit tests were missing so have been added. Swift name differs from default translation so has been provided via NS_SWIFT_NAME.

The change was only applied at the top level. A separate change will be required at the Configuration level.

Changeset

M	CHANGELOG.md
M	Source/Bugsnag.h
M	Source/Bugsnag.m
A	Tests/BugsnagTests.m
M	UPGRADING.md
M	iOS/Bugsnag.xcodeproj/project.pbxproj
A	iOS/BugsnagTests/Swift Tests/BugsnagSwiftTests.swift

Tests

Added ObjC unit tests and a Swift UT to check Swift name exposure.

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

[kattrali]

Tests which notify should start Bugsnag with a configuration which disables delivering reports in the test, either by changing endpoints or adding a beforeSend block to reject them. Looks good otherwise.