Setup Changes

Don't nest under Board, change constant references to class methods.
Add debugger to gems.
Change the way BoardFiles accesses the files.
Fix Board square setters.
Add factory girl, mocha, and some factories.
Fill in Pawn class and Piece behavior.
Add yard.
Add specs and documentation for BoardFile. Make it responsible for returning EmptySquares.
Minor changes to BoardFile documentation.
Make EmptySquare instances equal one another no matter what.
Add BoardFiles spec and documentation.
BoardReferences module, BoardCoordinate changes, BoardHelpers

- BoardReferences module to DRY up references to file names and rank
names.
- BoardCoordinate specs, more documentation.
- BoardCoordinate methods to_s and rank.
- BoardCoordinate#relative_coordinate_set limit option.
- BoardHelpers for specs to easily generate coordinates.
Piece: Equality testing checks for class first to avoid impossible method calls.
Board: Improve documentation, add #coordinates_of. Add be_empty_square matcher.