Implementing feature 'Get list of collections for a given project' for issue #26

Feature
---
As a user I should be able to get the list of event collections for the project set.

Implementation
---
As a reference you can take a look at the event resource https://keen.io/docs/api/reference/#event-resource

```ruby
Keen.event_collections # => [{ "name": "purchases", "properties": { "item.id": "num", ... }, ... }]
```