Merge pull request #103 from mirochaj/master

Added documentation for SDSS query tool.
documentation related fixes for sdss to try to get the sphinx docs to
build
Merge pull request #104 from keflavich/sdss_docfixes

SDSS docfixes
Moved alfa to alfalfa, cleaned up alfalfa/core.py (separated cross-identification and spectra download routines), small docstring change in sdss/core.py, and added alfalfa.rst to docs.