Remove duplicate references

The BlocksKit.h file was added twice to the Xcode Copy Headers build phase.

This change should fix the Xcode warning caused by it: he Copy Headers build phase contains duplicate references for one or more files"
Merge pull request #219 from vially/patch-1

Remove duplicate references
Update README.md
Podspec update
Restore NSURLConnection(BlocksKit).delegate; fixes #220
Always define a2_methodSignaturesCompatible
Replace libffi usage with pure NSInvocation
Remove libffi/ffi-mini
Ignore IntelliJ metadata
Return invocation results as NSValue
Simplified A2BlockInvocation initializer; rewrite tests
Document constant
Fix @encode(SEL) formatting
Fix whitespace
Copyright notice fixup
Update podspec for 2.2.2
Cut unit test exec time
Fix rare truncation issue in dict tests
Fix async unit tests on single-threaded devices
Fix mail compose test on devices w/o mail accounts