RSpec Web Mocks For Merchants Class

Hi,

I added some [web mocks](https://github.com/bblimke/webmock) to the `merchant` specs to help decoupled the examples from the needs of an actual running web server. 
In a nutshell, with the mocks, 
1. The specs can be run without a web server.
2. The REST endpoints, and customize the HTTP responses to be changed at-will. All the stub functions can be found in `spec/finances/merchant_stub_requests.rb`.
3. The specs to be freed from cumbersome set-ups in the `before` hook and tear-down in the `after` hook. For example, there is no need to invoke `create` while testing the `all`, because all we really care about is whether `all` can properly retrieve and deserialize the responses, given some URL and the necessary parameters.

Let me know if this is helpful. Otherwise, feel free to delete this PR.