Juniper-specific commit operations

add support for ‘commit comment’, ‘commit synchronize’, and ‘commit at’
commands by extending the Commit class to a juniper specific method
within the third party juniper rpc.py file.
confirmed and at_time mutually exclusive

Added in error checking to make sure that an error is kicked back when
the user sends both commit confirmed and commit at. Junos cannot accept
both of these on the same commit.
Merge pull request #32 from NetworkAutomation/master

Add Juniper specific commit options (commit comment, commit synchronize, and commit at)