Getting rid of legacy `history` field in database entries.

This field does nothing useful and only creates useless commit differences
on github.  Its removal lets the database be cleaner.
For kinetics databases, write reference and referenceType fields only if not None.
Merge pull request #197 from connie/universalDatabase

Universal database progress.
Looks good to me, and I merged the RMG-database commit too.
(And after all that effort that went into preserving the history entries and detecting changes....)
Remove unused `recommended` flags for databases.
Makefile checks that you have PyDAS and PyDQED on your Python path
Change URLs of PyDAS and PyDQED to GreenGroup in installation instructions.

Hopefully Josh will alert us to any improvements he makes, now that he has left.
I also switched to https instead of git, so that you don't need a github account
and login etc.

I also tweak the linux installation instructions.
Just formatting, really.
Change the default location of the database (I think this is more likely right)

It's very confusing to me where this is set, and in what priority.
(eg. the .rmgrc config file). I'm not sure why the prior one seems to work in other places
but not on Travis. Perhaps the $RMGpy environment variable?
Add work_in_progress decorator for currently failing Unit Tests

This is a proof (test) of concept. If it works, then we should add this
decorator to all the unit tests that we currently EXPECT to fail.
That way, the Travis-CI build test will pass - the failing tests will
just be skipped if they fail (still with an error message).

We shouldn't forget to go and fix the tests though!