Split browser option into flag and option

As noted in #108, the ``--browser`` option is currently broken if no
argument is supplied. Split the argument into two: a flag
(``--browser``) indicating if a browser is to be opened and an option
(``--with-browser``) which may optionally be used to specify which
browser is to be used.

Fixes #108.

You may now optionally pass a browser name or executable to the
--browser option, which is then preferred over the default browser.

See #107.

Add option to auto-launch browser on server

Add command-line argument --browser (shortened to -b) which will
auto-launch a web-browser pointing to the Klaus server via the Python
webbrowser module.

Note that we do things in the wrong order in that we launch the browser
and *then* start the server. In the absence of a cleaner way of getting
a function to run after server setup this approach actually works in
practice. If this doesn't Just Work for someone we can think again about
more complex options.

The 'git' executable may very well be absent on other platforms as well.

catch WindowsError

Thanks to @Mechazawa for the report and patch.

Thanks to @lb1a for the patch.

Fixes #87.

Thanks to @lb1a for the initial patch.

Fixes #35.

Handle htdigest file properly in contrib.wsgi

Fix #90

Fix #90

Fix #90

See https://github.com/jelmer/dulwich/issues/144

Thanks to Klaus Alexander Seistrup. Fixes #89.

If lexer can't be guessed from the file name, try to guess it from only
the file contents. Fall back to no highlighting if this fails too.

Thanks to Daniel Krüger for the report.

See: https://groups.google.com/forum/#!topic/klaus-users/OKyd4suHN2g

.m files may be both Matlab and Objective-C. Pygments already handles
these cases gracefully; not sure why didn't use it in the past.

Thanks to Gnewbee for the report.