Accept dot-delimited schemaname.tablename 

Presently, blaze.SQL isn't accepting <schemaname>.<tablename> to identify tables outside the connecting user's own schema.  Parsing out a dot-delimited prefix and passing it to SQLAlchemy as "schema" fixes that.

I apologize for not including tests, but I haven't been able to get the tests to run.  Hopefully the small size of the pull request makes that OK.
Suggest the use of `rsplit`

```Python
In [2]: 'a.b.c.d'.rsplit('.', 1)
Out[2]: ['a.b.c', 'd']
```