Add option to prefix values in xcconfig files

Passing a prefix will prepend a string to every configuration value 
in the xcconfig file.  For example the configuration value:

	OTHER_LDFLAGS = -framework Foundation

Passing prefix PODS_LIBCOOL_ results in:

	PODS_LIBCOOL_OTHER_LDFLAGS = -framework Foundation
Add xcconfig config value prefix test
Merge branch 'master' into xcconfig-prefix

* master:
  [Travis] Try to get Ruby 1.9.3 build to work again.
  rewrote specs with native asserts
  simplified schemes related code in Xcodeproj::Workspace
  added specs for testing schemes related functions
  - refactored schemes loading code - fixed path related bug from previous implementation
  added schemes property to Workspace (loads all schemes from workspace)
  Fix Readme typo
  Release 0.6.0
Merge pull request #70 from CocoaPods/xcconfig-prefix

Xcconfig prefix