typo in ParseKit.podspec

typo prevents source files from being included
add exclusions to ParseKit so it will compile on osx and ios

* MGTemplateEngine isn't used on iOS at all
* RegexKitTemplateMatcher isn' used at all (and also fails because of an import to misnamed file)
* PKSParserGenVisitor is OSX Cocoa only.
Merge pull request #2588 from demosdemon/patch-1

typo in ParseKit.podspec