Feature/test coverage

Added [SimpleCov](https://github.com/colszowka/simplecov) tool for test coverage of our Platform. 
In file - `simplecov_helper.rb`, we can restrict sections of app which we don't wish to be covered, because of side-effects of system based commands and processing.

Apart from them, I am also thinking to restrict [`lib/command.rb`](https://github.com/amahi/platform/blob/master/lib/command.rb), [`lib/partition_utils.rb`](https://github.com/amahi/platform/blob/master/lib/partition_utils.rb) and few other files which we may not hit the DB at all.

We can discuss about it here, and further configuration can be done before merging. 

**To run:**
- `bundle install`
- `rake spec`
- Open `/coverage/index.html` in your browser. 


Cheers!