Merge pull request #190 from kurtwalker/MONDRIAN-1771

MONDRIAN-1771 - adding extension point for JdbcSchemaFactory, additional point for segment loader.
[MONDRIAN-1772] Set the Table.Column constructor to be public so external implementations of JdbcSchema can create them.
Merge pull request #191 from mkambol/column

[MONDRIAN-1772] Set the Table.Column constructor to be public so externa...
MONDRIAN-1771 - some cleanup.  removing isMongo and a couple fixme's