This change makes a few improvements to rbext to modernize command

line parsing, help output, and console output.
rbext --help and each subcommand's --help now provides more useful

instructions, with links to useful URLs where appropriate. They're

formatted by the new HelpFormatter split off from rb-site.
rbext --version now works, showing the Review Board version, Python

version, and Review Board install path.
rbext test has new native support for -x, --pdb, and

--with-coverage options, and explicit documentation on how to make use

of nosetests options.
Console output uses the new Console object, which doesn't do a whole

lot in our case, but does allow us to standardize things.
Variables for manual URLs were also standardized.

Checked rbext --help, rbext create --help, and rbext test --help

and verified the output was formatted correctly.
Ran rbext create and verified all the generated files were correct,

including URLs.
Ran rbext test with the new options and verified that they impacted

the tests I ran.