BibClassify: case insensitive taxonomy

* Fixes a bug where the look-up of ontology names from the
  taxonomies were forced to lower case.

Signed-off-by: Jan Aage Lavik <jan.age.lavik@cern.ch>
Just add a docstring here. You can remove the rest of the implementation. See:
```ipython
In [1]: class TaxonomyError(Exception):
   ...:     pass
   ...: 
In [2]: raise TaxonomyError("foo bar")
---------------------------------------------------------------------------
TaxonomyError                             Traceback (most recent call last)
<ipython-input-2-53f69365ca4d> in <module>()
----> 1 raise TaxonomyError("foo bar")

TaxonomyError: foo bar
```