Repair formatting throughout documentation

Repair formatting throughout documentation

1. Section decorators to [one style](http://docutils.sourceforge.net/docs/ref/rst/restructuredtext.html#sections):
without inset at both side and with same length as text.
2. Fix broken [reference](http://docs.sqlalchemy.org/en/latest/core/type_basics.html#generic-types).
3. Convert tabs to space in some small files.
4. Some python code snippets have python+sql syntax hint.

Change-Id: I39a7a41ef0b0591c6bf1e610748e2b5c19fc5379
Pull-request: https://github.com/zzzeek/sqlalchemy/pull/361