Implements TSVECTOR type for postgresql.

Signed-off-by: Noufal Ibrahim <noufal@nibrahim.net.in>
Updates documentation for tsvector type.

Signed-off-by: Noufal Ibrahim <noufal@nibrahim.net.in>
Adds tsvector to ischema_names for reflection to work.

Signed-off-by: Noufal Ibrahim <noufal@nibrahim.net.in>
Adds test to verify tsvector creation.

Signed-off-by: Noufal Ibrahim <noufal@nibrahim.net.in>
Merge branch 'tsvector' of https://bitbucket.org/nibrahim/sqlalchemy/branch/tsvector into tsvector
- round trip test
- changelog
- some doc rearrangement