#79 Generate links to bug reports from @bug field

[MarkRoddy]

1) Add an optional bug number argument for the @bug field.

2) Add a command line option to specify a base URL to a bug tracking server. For instance:
epydoc --bug-url=http://server/cgi-bin/bugzilla/show_bug.cgi?id=XXX

3) Use the bug number and base url to generate links to the bug reports in HTML documentation.