Add support for CODEC column option

[athre0z]

Continuing the discussion from here, this implements the required changes to add support for the dialect-specific CODEC(...) column option. Thank you for the hints regarding the implementation, they proved helpful in getting this started!

I got a little bit confused by your original message mentioning visit_column and realizing that this method only generates the column name, however looking around in the MSSQL implementation revealed that overriding get_column_specification appears the be the way to go. I never thought that MSSQL would prove to be helpful for anything in my life, but I stand corrected.

In the edited comment you changed it to visit_create_column, which I tested and can confirm that having the code in this method would also work. This PR currently implements the get_column_specification approach, but I could just as well move the code down a few lines if you'd prefer that -- just say the word.

Also, would you want to advertise this feature in the README?

Fixes #88

[coveralls]

Coverage increased (+0.09%) to 93.939% when pulling 433c300 on athre0z:feature/column-codec into d4be4a8 on xzkostyan:master.

[xzkostyan]

Excellent PR!

Yep, you can add an example of codecs usage in section SQLAlchemy declarative support.

[athre0z]

I added a column with codec overrides to the existing README example and, since I assume that these aren't covered by CI testing, verified that it is still working by pasting it into a project of mine. Seems to be working fine!

(I renamed it to RateXX since Rate was already in use :D)

[xzkostyan]

This requires SA>=1.3