refactored element code into elements sub-package
making sure there is only one instantiation of an elmenent per task graph instance
caching the task_graph instance on a per-run basis
storing state information on the elements only
renamed elements -> resources
fixed some bugs
moved around the resource linking to be after all of the tasks have been initiated to resolve #34
got --backup to work with aliases in depends statements
Merge pull request #34 from deanmalmgren/element-refactor

Depends does not match a previous alias