Implement the ability to specify schema for PostgreSQL #106

This pull request allows the user to set the schema that a table lives in when building the TableMaps.  From what I can tell it's only relevant to PostgreSQL, so the new interface method on sqlite and MySQL acts exactly the same as the old one.  

Sorry for the merge, I needed to begin to use the change immediately and go won't let you specify the branch to pull.  If I should avoid pushing changes to master in the future to make this process easier please let me know, I don't often contribute using Github pull requests.

All tests that pass on the current master pass in my branch as well, although this functionality is currently lacking tests outside of my own usage.

The test that is failing on master is:

--- FAIL: TestWithTimeSelect (0.11 seconds)
	gorp_test.go:1233: 2 != 1