Separate gemspec from Rakefile

Separate gemspec from Rakefile

The benefit of this separation is that hosting services such as GitHub
will automatically build a gem every time the version in the spec is
changed. The underside of this separation is that hosting services such
as GitHub will automatically build a gem every time the version is
changed.

To allow the automatic gem generation to trigger on the release of
0.9.15, we demoted the version in the spec to 0.9.14. This gimmick is
needed because the gem spec doesn't allow non-numeric versions.

Since the gemspec is now standalone, it uses Dir instead of FileList,
which is equivalent for our use case. Moreover, the Rakefile only
defines the gem tasks if the spec is present.