Fix RKMappingSourceObject init method warning

Instance variable used while 'self' is not set to the result of '[(super or self) init...]'
Fix potential nil value in dictionary
Merge pull request #2098 from 0xced/fix-analyzer-warnings

Fix analyzer warnings
[Podspec] Use __has_include to check for prescence of CoreData subspec
[README] Link to CocoaDocs for SOCKit

Closes https://github.com/RestKit/RestKit/issues/1884.
[README] Update iOS Deployment Target