Add SQL Server support

Because I really like RedBean, I added SQL Server support.
You said that you won't add nor support, but was willing to help if someone tries to do it, so here it goes. Off course, I don't expect you to give support or test for new features, but I'll try to keep this up to date.

I'm not sure how this affects RedBean 4, so not sure if you should add the PR - I leave this to you =p

Maybe still needs some work, but it's creating, reading, updating, deleting tables, references tables and foreign keys.

Tested on SQL Server 2008 R2.

PHP Driver for Windows
http://www.microsoft.com/en-us/download/details.aspx?id=20098

Setup
`R::setup('sqlsrv:Server=localhost;Database=dbname', 'user', 'password');`

Again, thanks for making RedBean.