Added a RHESSI flare list object

This is in response to #1018. In the future more methods could be added to this object to increase its functionality. In summary:

  - Added a RHESSIFlareList object within sunpy.instr.rhessi
  - RHESSIFlareList contains .flare_list (the full list of RHESSI flares parsed from an online file). Each entry in .flare_list is a dictionary. It also contains .meta, which contains some useful information from the footer of the file.
  - Also added the method find_events_by_date_range to search through the list by a date range
  - Added tests.