Save operation

This is work in progress task for adding "save" operation into atmosphere. We agreed that "save" operation will only replace VMTs with new saved VM. It will not update information about AT `name`, `description`, `port_mapping_templates`, `endpoints`, `initial_configurations` and this information will be presented in the API doc.

Before it can be merged into master following actions need to be finished:

  - [x] VMT versioning
  - [x] migration adding version into VMT should fill this value with number 1 for all VMT managed by atmosphere
  - [x] Save action
  - [x] Remove VMT older than version
  - [x] Monitoring task sets correct version when VMT is a copy of another VMT (such situation occurs during VMT migration into different compute site)
  - [x] REST interface invoking Save action
  - [x] Save REST API documentation

Fixes #13 
This class and the rest of classes mentioned by you are implementation of Action pattern. This pattern is quite simple - name should explain what class does and it should have only one method `execute`. Maybe classes mentioned by you should be moved into `app/actions` directory, but for sure I'm against changing the name to end with `er` or `or`.