Add 256 color support to syntax highlighting

This pull adds 256 color support to pudb through urwid. Authors of syntax files can simply use a foreground color > 16 in place of named colors.

The urwid issue and workaround around on https://github.com/kemist/pudb/blob/master/pudb/theme.py#L301 is caused by urwid attempting to shoehorn a color > 88 into an 88 color palette, which throws an exception :( (see  https://github.com/wardi/urwid/blob/master/urwid/display_common.py#L824).

My own color palette as an example https://dl.dropbox.com/u/330001/pudb_ingelmo_theme.py

I love pudb!

![vim_next_to_pudb](https://f.cloud.github.com/assets/348608/213591/a1db60ac-8395-11e2-833a-63326828ed52.png)