Part of issue64.

Merged and pushed to the private repository.

I skimmed over the code and didn't spot any obvious mistakes. Can be merged from 
my point of view.

Jendrik can you have a look just to check that I didn't mess anything up?

https://bitbucket.org/flogo/downward-issues/pull-requests/15/issue625/diff

I renamed ContextEnhancedAdditiveHeuristic and LandmarkCutHeuristic (named after
the class) to cea_heuristic and lm_cut_heuristic (named after the file).

We decided to change the naming convention for namespaces to use lower case
names (like file and directory names) to avoid classes with the same name as
their namespace. Such classes lead to problems with forward declarations, e.g.,
we cannot use "using Options::Options;".