Support negative number literals
Be more Django-y about undefined values

Previously evaluating undefined variables threw an error. Django
doesn't do this, so we won't either. Instead evaluate to the empty
list, which conveniently enough works both as an emptry string
and as an empty array
Rename variables for clarity
Support number literals in custom tag calls
Support {% for .. in .. reversed %} syntax
Avoid problems with rebar eunit using the TEST macro. #69
Merge pull request #73 from okeuday/master

Issue #69 fix
Added in a newer rebar, and the pmod_pt parse transform.
Added a few tests of with with variables/attributes.
Merge pull request #75 from davidw/pmod_fix

Added in a newer rebar, and the pmod_pt parse transform.
Merge pull request #76 from davidw/with_problem

Added a few tests of with with variables/attributes.
Remove pmod_pt dependency