Add the ability to use multiple counters

[rofrano]

As a User
I need the ability to create multiple hit counters
So that I can keep track of multiple things

Assumptions:

• The name of the counter to use should be pass in by the user
• A call to GET /counters should list all counters
• A call to GET /counters/foo will operate on the foo counter

Acceptance Criteria:

When I call GET /counters
I should see a list of counters as a JSON string
When I call GET /counters/foo
I should get the current counter value for the foo counter
When I call POST /counters/foo
It should increment the foo counter and return he current value