Tabletop - Table, Specs added, README updated

- Added Table class having method to place Robot on Table when
co-ordinates are valid. Also a method to return the co-ordinates if the
Robot is placed.

- Added tabletop_spec having the tests to verify possible positive and
negative tests for each method added

- updated README file with PLACE
Rakefile Updated

- updated Rakefile to consider all spec files in specs folder
Robot Spec file updated for method name

- updated Method names to standard name to point to method name in
Robot class