Added modular DAL, updated gluon imports to new structure

Updated to commit: f17493b52c34db33fb641974d5d6a3d8926fbafd
This replaces (already closed) PR https://github.com/web2py/web2py/pull/483, adding correct imports from gluon.

A modular version of DAL. Please check https://groups.google.com/forum/#!topic/web2py-developers/0B8Bg_Evmj4 for reference.

Notes:
* Missing latest PEP8 commits, @mdipierro I'm sorry about that, we should re-validate to PEP8 after merge.
* Missing test_all() function as I don't know how you want to manage tests with this modular version
* Tested with sqlite3, **needs debugging**!