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
Merge pull request #191 from ethanwhite/readme-update

Minor cleanup of README