Merge branch 'release/v0.20.0-rc1'
Merge branch 'release/0.20.0'
Drop pre from README.md install instructions
Merge branch 'release/0.20.1'
Adjust version and tag in Podspec for release
Merge branch 'release/0.20.2'
Merge branch 'release/0.20.3'
Merge branch 'release/0.21.0'
Update CocoaPods to 0.28.0
Bump Pod dependencies
Add support for serializing values to null. closes #669, closes #1311
Bump TransitionKit ref
Fix test breakage from inclusion of milliseconds in ISO 8601 date output
Add TKTransition to the project so static library and framework will build standalone
Fix warning
Conditionalize tests for OS X
Switch to release version of xctool to fix Travis build errors
Merge branch 'release/0.22.0'
Enable `GCC_WARN_64_TO_32_BIT_CONVERSION` warning
Respect `assignsDefaultValueForMissingAttributes` when mapping `nil` values

* Set `assignsDefaultValueForMissingAttributes` to `YES` for request mappings to enable serialization to NSNull