Improve SDSS API compliance

This PR moves the SQL query construction into a ```_args_to_payload``` function.

The ```get_spectra``` and ```get_images``` functions now accept coordinates. They also accept the result of the ```query_region``` as before, but it has to be assigned to parameter `matches`.

```get_spectra``` can also be called with plate, mjd, fiberID values directly. ```get_images``` can also be called with run, rerun, camcol, field values directly.

The PR also includes documentation and tests for new ways of calling the functions.

Overall, this makes the module be more compliant with astroquery's API specifications.