Capistrano integration

This PR implements deploy tracking with capistrano.
Simply add to `deploy.rb`:
```ruby
require 'rollbar/capistrano'
set :rollbar_token, 'your token'
```
It will send POST to the deploy endpoint (via Net::HTTP) after each deploy.

Available options:
* `rollbar_token` - project access token, required
* `rollbar_env` - deploy environment, `rails_env` by default

For  `capistrano/multistage` it can be useful:
```ruby
set(:rollbar_env) { stage }
```