OS X target support with AlamofireOSX.framework product.

* Added OSX target - generate framework for OSX 10.10 target.
* Tweaked DownloadTests.swift to avoid deleting entire OSX Documents directory during tearDown()
* OSX Tests passing with the exception of AlamofireDownloadResponseTestCase.testDownloadRequest()
   * Downloaded file is saved with **.json** extension instead of expected **\\(numberOfLines) lastPathComponent**.