ec2/connection.py - add filter parameter to trim_snapshots()

This patch allows you to filter the trimmed snapshots. In some cases you may not want to trim all snapshots in an account or you may not have the same retention requirements for all snapshots. With this patch you can filter the snapshots to be trimmed by a tag.

This is a backport of https://github.com/Kixeye/boto/commit/4957b70 on the "develop" branch.