Merge pull request #2 from github/master

Sync github/Mantle
Add system requirements and installing information
Improved formatting in README
Merge pull request #139 from akashivskyy/readme-installing

Add installation guide to README
Fixing missing instancetype on 10.8
Merge branch 'master' into opt-out
Renaming usesImplicitJSONMapping
Improving +JSONKeyPathsByPropertyKey docs
Fixing documentation
Refactoring +enumeratePropertiesUsingBlock