Update information on OS X binaries

Since 1.6.0 we are successfully (at least for the time being)
supporting OS X, so no need to caveat the binaries.
Remove MySQLdb as an optional dependency

We switched over to only using PyMySQL a while back so we shouldn't
list this as a dependency.
Remove old reference to Database Toolkit

And also simplify this line by not mentioning main.py
Fix the wxpython check in setup.py

The correct import statement for wxpython is 'import wx' not
'import wxpython'.
Make sure that CITATION is added to distributions

This adds a MANIFEST.in file to make sure that CITATION is included
in distributions. This should be handled by the data_files line in
setup.py, but that isn't working. This was apparently a bug in
Python 2.6 that was fixed, but there are reports of continued
issues in 2.7 http://stackoverflow.com/a/2998311/133513.
Merge pull request #191 from ethanwhite/readme-update

Minor cleanup of README
Merge pull request #192 from ethanwhite/fix-wx-check

Fix the wxpython check in setup.py
Merge pull request #193 from ethanwhite/fix-pypi-install

Make sure that CITATION is added to distributions