created Evaluator class WIP
Evaluates expressions with +-*/ and cell names.

Not tested yet >_>
tested the Evaluator. It seems to work. Does "+,-,*,/" operations, counts dependencies and catches most errors.

Having some trouble with git-hub, so not sure what is getting pushed in the end orz.
Cell definition and working evaluator w/ provisional dag.
Merging Evaluator and Cell classes with the DAG class.
Separated Cell class into separate h/cpp files for better flow control.
Also updated Evaluator class to account for the correct DAG functions.