Add the query_url method

Closes #38 (Expose a method that turns parameters into a query URL)

Overview
-
Expose a method that turns parameters into a query URL, so the URL could be used to query asynchronously or from another HTTP library or system.


Solution
-
Add a `query_url` method that generates a URL for a query without running it, and allow the `query` method to be a consumer of this one.
```ruby
Keen.query_url('users', {analysis_type: 'count'})
# https://api.keen.io/3.0/projects/12345/queries/count?event_collection=users
```