refactored RubyGrid::GridBase#is_valid?

 - Moves Fixnum check to a new private helper method RubyGrid::GridBase#is_number?
 - #is_number? does not check class to be more duck-type friendly, checks whether
   given object responds to selected methods.
Refactored RubyGrid::GridBase#get_cells(cells)

 - Changed checking cells' if cells class is Array to duck-type friendly check
 - Added comment to spec/spec_helper.rb
Refactored RubyGrid::GridBase Fixnum checks to use #is_number?

 - Refactored RubyGrid::GridBase#populate(data) to not check data's class
Renamed RubyGrid::GridBase to RubyGrid::Grid
added cell class
Revert "added cell class"

This reverts commit 5090f7b5b3682eb236ef9d9d71d3f7a85743f085.