Implement bitpos command.

This command landed in unstable today.
Tests are taken and translated from the original Tcl test suite.
Handle only stop parameter set, but not start.

If start is not specified, the value given for stop will be injected
into start's position; it is wise to safeguard against this.
(via @yaauie)
Merge pull request #412 from badboy/bitpos

Implement bitpos command.