Refactor for efficiency

[mjpieters]

This is a larger refactoring that avoids re-creating objects each time there is a traceback to print.

• Move imports to module level
• Make most object definitions dependent on pygments being available
• Create a lexer and formatter just once
• Determine if colorama support is available just once

It also incorporates pull requests #9 and #11, so could be seen as a replacement for those.

[staticshock]

Thanks!