[wip] better reverse engineers the mapping metadata from a database

# The problem:
1.use symfony2 doctrine:mapping:import from msyql database to annotation
2.use symfony2 doctrine:schema:update will see a lot of sql update about just import database schema.
3.this PR fix eliminate some of those sql update...

# has fixed problem:
* column default
* column unsigned (add test)
* column comment
* column type decimal with precision and scale
* table with simple index
* table with unique index