Refactors data handling classes. Datasource now handles query parameters.

Datasource class methods altered to take id, field, and include arrays.
Upon reception of said arrays, they are converted to a String parameter
that Retrofit can interpret with the @Query parameters. The CourseraAPI
class replaces previous methods with methods that take string id, field,
and includes parameters. This allows for smaller packets of data to be
fetched rather than all at once.