Make MSSQL DBAPI selectable

[paj]

The MSSQL module supports three different DBAPIs. It should be possible for the user to specify which one they want, ideally using a string, so this can be done from a framework's configuration file. Although there's code that tries to support this, it doesn't work at present, primarily because the MSSQLDialect.new doesn't see the constructor arguments, which would tell it which DBAPI to use.

Fixing this may require a bit of a refactor, possibly beyond the MSSQL module.

[Mike Bayer]

this will be eventually subsumed by #672. URLs will be of the following form:

pyodbc:mssql://<connect>

however id also favor an interim solution to get the above syntax working more quickly.

[Mike Bayer]

the existing module argument should work for this for now, the full feature is the dialect refactor for 0.6.

[Mike Bayer]

Removing milestone: 0.5.xx (automated comment)