correct start of month in trim_snapshots docstring

Monthly snapshots are from the first day of each month, whereas the
old docstring indicated they were from the first Sunday of each month.
add trim_snapshots option to limit number of monthly backups
Add unit tests for trim_snapshots defeault behavior and test the new monthly backups feature
Merge pull request #1688 from danielgtaylor/andrewyates-develop

[EC2] Add monthly_backups argument to trim_snapshots. Fixes #1688, #1612.