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 
I don't like this method name. First of all at used in this context is confusing. I'm affraid that this name should be more elaborate otherwise it's difficult to guess what it does. It's body is not self explaining. I get the feeling that this method could be implemented in an easier way, in an arel query for instance. I know this a personal preference so I don't insist on changing method body.