Add failing unit tests for bitwise and, or, and xor

I decided to exclude bitwise NOT. This operator applied to a decimal values is
just the compliment, which I don't see an obvious use for.
Add support for bitwise and, or, and xor
Explain why bitwise assignment is different

In python, arguments to bitwise operations must be either plain or long
integers.  The cast is required for correctness and for the unit tests to pass.

This could be improved slightly by verifying the arguments are in fact integers
before the operation is performed.  As it stands "1 & 3.1" will round down 3.1
to 3 instead of throwing an error.
Add associativity tests for bitwise operations
Add bitwise operations to documentation
Merge pull request #7 from jbernard/master

Add support for bitwise AND, OR, and XOR