Allow get_note to fetch previous versions of a note
Change from using `has_key()` to `key in d`

Since `has_key()` has long been depreciated.
Correct timestamp for modifydate

It should have been in seconds all along. Never milli-seconds.

Since this is an API have to be careful not to override a value if the
client has already set it.
Merge branch 'master' of https://github.com/atomicules/simplenote.py