*** Individual commit messages included below ***
* Preliminary support for bitwise operations as infix operators. Tests to follow.
* Added bitwise xor, shift left and shift right operators
* Fixed the BitwiseOr class so it uses the :| operator instead of :&
* All the methods for the bitwise operators in the Arel::Math module now wrap them up in Arel::Nodes::Grouping so the operation becomes isolated like addition and subtraction
* Preliminary set of tests for the new operators
* Updated README with examples of bitwise operations
* Added a new UnaryOperation class which is a riff on the InfixOperation class
* Added tests for UnaryOperation (inspired by InfixOperation tests)
* Added the bitwise not (~) operator as a UnaryOperation
* Added tests for the bitwise not operator
* Added documentation for the bitwise not operator
* Updated gemspec using `rake arel.gemspec`
