New `Translator` class for Trac markup to Github markdown translation.

The `Translator` class replaces the `convert_text` function. This
requires the regex to be compiled only once and reduces the number of
arguments that are passed at every text conversion. Other changes:

 * Documentation for using `token` in place of `user` and `password`.

 * Documentation for converting Trac markup to Github markdown.

 * `NullTranslator` just returns the same text that was entered.

 * `getConfigOption` can take Boolean options.

 * `tracurl` -> `trac_url`

 * `convert_text` config option determines whether to translate the
   text in the tickets.

 * `convert_text` function removed.