Work around a bug in pandas.DataFrame.replace to set the color

In the case a original value (e.g. 0-9A-F) was part of a color
defintion, using df.replace{dict} would do a double replace on the
same cell, resulting in colors like "#12345#ABCDEF", which would
then throw an error during plotting.

Using apply(lambda x: replacements[x]) is probably slower, so should
be replaced when the pandas bug is gone.

See https://github.com/pydata/pandas/issues/5338 for the bugreport
in pandas.
Fix facet_wrap error

Using facet_wrap was throwing an error about index out of range.
Add error checks to facet_grid

facet_grid doesn't work without both x and y mapping, so error out if
they are not available.
geom_density: check for float(x) and special case Timestamps

The column which the x mapping points to, needs to be convertable to
float (gaussian_kde), so check that this is the case.

Special case Timestamp columns, but that probably needs more work.
New Tests

Add more tests. The Images are not included as I currently do not have
the theming available and so the images will lead to failures in a
themed environment.

All test at least do not throw any exception, but the images do not
look good in most case and should be checked if the tests make sense
or the correspondig code needs improvements.
Merge pull request #80 from JanSchulz/new_tests

New tests