Fix bug introduced yesterday

The tupak.core.prior call in the core/__init__.py file was accidently
removed during testing. This puts it back in.

tupak/core/__init__.py

 from __future__ import absolute_import
 import tupak.core.likelihood
-#import tupak.core.prior
+import tupak.core.prior
 import tupak.core.result
 import tupak.core.sampler
 import tupak.core.utils