Switch from using passwords and user names to tokens.

Using tokens for added security. Also tickets can have the different
owners rather than just the main user's login. Tokens will have to be
gathered for each user.
Github issue comments have the correct Trac author.

 * Include link to the original ticket.

 * Converting text from TracWiki to TracWiki2MediaWiki perl script.

 * Converting MediaWiki text to Githhub Markdown using Pandoc's Python
   interface.
Function for creating ticket to issues map and regular expressions.

Function `_createTicketToCommentsMap` creates a ticket to issues
mapping so that `ticket:XXXX` links work correctly in GitHub.

The `convert_text` function uses regular expressions for converting
from Trac wiki markup to Github markdown. Stopped using Pandoc and
TracWiki2MediaWiki as this process was broken in many ways.
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.
Merge pull request #26 from wd15/fipy-migration

* Changed user authentication from password to token.
* Added basic translation of Wiki markup.
* Added conversion of ticket:xx links.
* Added backlink to from Github issue to original Trac link.

Contributed by wd15.
(Contributed by wd15.)