Update README

Merge pull request ishkawa#101 from ishkawa/feature/fix-Result-version

Specify the version of Result by ~> operator to avoid the collision

Merge pull request ishkawa#98 from ishkawa/feature/update-result

Update Result to 0.6.0-beta.6

Merge pull request ishkawa#97 from ishkawa/feature/fix-submodules

Update dependency to fix installing via Carthage

Bump Xcode version on Travis CI

Merge pull request ishkawa#94 from inamiy/access-level

Change access level of RequestType.parseData() to public.

Merge pull request ishkawa#86 from ishkawa/feature/session

Rename `API` to `Session` and reorganize relationship between `Session` and `NSURLSession`

Merge pull request ishkawa#84 from ishkawa/explicit-null-value

Improve nullable keys handling, restore [String: AnyObject] for `parameters`

Merge pull request ishkawa#83 from inamiy/nullable-parameter-value

Fix parameter type from [String:AnyObject] to [String:AnyObject?] for nullable value.

Update podspec