[SQL] Make Hive Metastore conversion functions publicly visible.

I need this to be public for the implementation of SharkServer2.  However, I think this functionality is generally useful and should be pretty stable.

Author: Michael Armbrust <michael@databricks.com>

Closes #750 from marmbrus/metastoreTypes and squashes the following commits:

f51b62e [Michael Armbrust] Make Hive Metastore conversion functions publicly visible.