Export DbMap.tableFor() and TableMap.columns

I'm writing a query language extension for gorp (currently at github.com/nelsam/gorp_queries, but the package name is subject to change still) and need `DbMap.tableFor()` and `TableMap.columns` exported for it to work.

This PR does precisely that, renaming `DbMap.tableFor` to `DbMap.TableFor` and `TableMap.columns` to `TableMap.Columns`.