Cleaning up compiler configuration in build script.
Fixing bad variable read crash. Including some relevent comments.
Adding destructuing variable assignment.

* Reworking Evaluator::Assign to provide additional features.
    * Moving original functionality to Evaluator::SetSymbol
    * Adding checking for valid symbols to Evaluator::SetSymbol using Evaluator::IsValidSymbol
    * Adding Evaluator::DestructuringAssign, which will assign a data grid into the names in the grid of an variable assign cell.
    * Turning Evaluator::Assign into a convience function.
    * Updating Grid::EvalGridCell to work with new functionality.
* Adding const where appropriate to large portions of Cell and any new code.
* Adding comments as I learned non-obvious information.
Including new example sheet showing new features at work.
Fixing some wxWidgets/GTK fixes.
Improving evaluation execution and management.

* Removing redundent execution pass.
* Treating variable reads like true data.
* Adding the ability to mark multiple cells at once.
Updating example to include new features.
Fixing a bug in selection mark.
Removing optimization that introduced bug.
Adding subgrid variable assignment. Fixing another crash.
Updating complex example to show features.
Fixing bug introduced by assignment from subgrid feature.
Merge pull request #6 from mason-bially/master

Destructruing Assign + Bug Fixes + Some Comments