Refactoring to reduce code duplication and dependencies

[bounav]

Fixes:

• Authorization URL no longer hard-coded to Mercado Libre Argentina! You can specify the mercado libre site you want to connect to.

Changes:

• Now using HttpClient instead of RestSharp.
• Leveraging message handlers to automatically refresh the access token when required. This reduces the code duplication in the mercado libre API class.
• Renamed MeliApi.cs to MeliApiService.cs.
• MeliApiService implements IMeliApiService (this enables easy mocking of API calls).
• Use of mock HTTP which means installing NodeJS to run unit tests is no longer necessary.
• json.net is used to serialize and deserialize JSON.
• Methods on MeliApiService are asynchonous (taking advantage of the HttpClient's asynchonous implementation).
• Use of HttpParams to create request parameters (and automatically URL encode its values).