Initial OS X package support, #317.

Basic support for OS X flat packages (.pkg)

- input/output
- output supports scripts, postinstall actions (using --info option),
  ownership option and bundle-id-prefix option
- requires pkgbuild (therefore OS X 10.7+ only), pkgutil for input
- no tests yet

ignore .DS_Store

ignore .pkg

identifier gets its own method

first few osxpkg spec tests

osxpkg: extract name and version from PackageInfo on input

osxpkg: rename option bundle-id-prefix to identifier-prefix

- 'bundle-id' is ambiguous, given the various 'bundle'-related logic possible with OS X packages

osxpkg: fix old 'osx_' attribute prefix to 'osxpkg' in osxpkg.erb

New option: --osxpkg-dont-obsolete, to add files to 'dont-obsolete' element in PackageInfo.

osxpkg: Define public/private methods.

osxpkg_spec: Tests for basic attributes through input/output
Merge branch 'feature/osxpkg' of https://github.com/timsutton/fpm into feature/osxpkg