Add a podspec
OS X target support.

Conflicts:
	Tests/DownloadTests.swift
OS X target support - include i386 and x86_64 archs.
AlamofireOSX.framework product added.
Alamofire.framework for both iOS and OSX.
Inherit code sign from project.
Merge remote-tracking branch 'mrackwitz/podspec' into podspec
Merge branch 'master' into osx-support
Merge branch 'podspec' into osx-support