DOC: Added entries into ecosystem.rst
Added some entries to ecosystem.rst.  Basically just searched Nullege for pandas.  There was a lot more, but thought this made a good start.

Had to rebase to avoid unrelated test failures in Travis.
 @jorisvandenbossche what do you think?
 @jreback I don't know any of these packages, so difficult to assess if they should be included in the list (if they are 'substantial' as noted in the text). 
 cc @dartdog any thoughts on these packages?
 closing...pls put on the mailing list and see if these are common; if so, reopen and will reconsider
