Adds support for SAVEPOINT, ROLLBACK TO SAVEPOINT, and RELEASE SAVEPOINT
Add support for time.Time objects to gorp.

This commit adds support for storing and retriving time.Time objects,
by passing through such objects to the underlying driver.  This
resolves gorp issue #14.

The commit is tested and works with the github.com/mattn/go-sqlite3
driver.

The commit does not currently work with the
github.com/ziutek/mymysql/godrv driver, due to a bug in the mysql
driver.  This problem is resolved by mymysql pull request #77.  I have
successfully tested that with the fix from pull request #77 applied to
mymysql, and with the current commit applied, gorp can correctly store
and retrieve time.Time objects from mysql databases.

This commit is NOT tested with the github.com/lib/pq driver.
add tracing for BEGIN/COMMIT/ROLLBACK

Traces the beginning and ends of transactions.
For PostgreSQL use "timestamp with timezone" as suggested by Joe Shaw
on github.
Merge branch 'Savepoint' of https://github.com/sqs/gorp into develop
Merge branch 'datetimefix' of https://github.com/seehuhn/gorp into develop
#14 - add Known Issues section warning MySQL users about time.Time support.  Disable TestWithTime() in test suite until mymysql tz patch lands
add SetNotNull separate from primary keys
Merge branch 'patch-1' of https://github.com/joeshaw/gorp into develop