First pass at tutorials

EJT: As originally written, this commit had large FITS files.  It has since
been modified to use `download_file`, and the fits files were removed from the
git commits to prevent requiring large downloads for those cloning the git repo.
But bear in mind that means all the commits from this one until `download_file`
was added won't work.
Finished a basic tutorial for fits images

Load the data, show it in linear and log scale, image stacking, and
writing fits files.
Finished a fits tutorial for fits tables

Also covers plotting 2d histograms
Cleaned up variable names and syntax

fits -> FITS
TEST_DATA -> image_data and event_file
some spaces removed
Now uses download_file from astropy.utils.data
harmonize notebook file name with directory
added published value to metadata
Manual merge of pull request #39 from eblur/master

Manual merge needed to expunge fits files from repo in favor of `download_file` mechanism and data.astropy.org