Modular DAL

A modular version of DAL. Please check https://groups.google.com/forum/#!topic/web2py-developers/0B8Bg_Evmj4 for reference.

Updated to commit: bac9039c6ea44c5ab6f09b1b8a2dd810a0e6afd8

Notes:
* Missing latest PEP8 commits, @mdipierro I'm sorry about that, we should re-validate to PEP8 after merge.
* Missing copyreg.pickle(<various classes>): I didn't understand why using copyreg (is for C extensions types isn't it?). Can be added after merge btw
* Missing test_all() function as I don't know how you want to manage tests with this modular version
* Tested with sqlite3, **needs debugging**!