Split targets and supporting classes into single purpose classes

Target class converted to PodTarget and SpecTarget representing Pod 
integration targets and podspec targets.
TargetInstaller converted to PodTargetInstaller and 
SpecTargetInstaller to handle variations in installing Pod targets 
and spec targets.
XCConfig generator converted to PodXCConfig, PublicSpecXCCconfig and 
PrivateSpecXCConfig classes to handle generating the different 
xcconfig files.
Requires Xcodeproj file changes to support namespaced xcconfig files.