Feature: Command prl-snapshot

This pull-request consider merging to 'develop', not 'master' (since we decided to follow 'git-flow' way )
So, there is implementation of #32. 

```
 vagrant prl-snapshot take [vm-name] [NAME]            # take snapshot, labeled by NAME
 vagrant prl-snapshot list [vm-name] [--tree]          # list snapshots
 vagrant prl-snapshot info [vm-name] [ID]              # show info about snapshot, specified by ID
 vagrant prl-snapshot go [vm-name] [ID] [--reload]     # revert VM to the snapshot, specified by ID
 vagrant prl-snapshot back [vm-name] [--reload]        # revert VM to the current snapshot
 vagrant prl-snapshot delete [vm-name] [ID]            # delete the snapshot, specified by ID
```
For help on any individual command run `vagrant prl-snapshot <command> -h`