Add an ImproperlyConfigured exception.

 - To be used to signal when a type is being used without a
   required library.
Remove explicit dependencies.
Merge pull request #25 from kvesteri/topics/optional

Remove explicit dependencies.
Added tests for TSVector autoloading
Merge remote-tracking branch 'origin/master' into topics/password

Conflicts:
	.travis.yml
	setup.py
Rename .raw to .hash