Use assert_raises instead of incorrect try / assert / except

Make Step.behave_as fail when calling an undefined step

Please accept smal fix related to skipped steps to xunit output plugin

I will document this xunit support as not-official until someone contribute with tests that simulate the jenkins parser or I have time to work on it.

I don't use the xunit output at all, so I can't even ponder on it

the tag support IS TO BE IMPLEMENTED through a TDD process

fix for #163

Code style improvements

A few more docs improvements

- Insert blank lines where missing
- Remove surplus blank lines
- Remove some unused imports
- Remove the use of the ``string`` module from lettuce.strings (work on the ``str`` objects directly which is just what ``string`` does - it's mostly obsolete now)
- Reshuffled a few bits of code to use ``all()`` or ``any()`` where appropriate and *not* use them where inappropriate

No functionality changes. (All tests still pass.)

Question: should it be .rst and thus be included in the docs? I think the answer is yes.

Corrected ***foo*** (which made "*foo*" in bold) to ``foo`` (monospace) in docs/reference/cli.rst.

Corrected `foo` (which is the default role, which is plain text) to ``foo`` (monospace).

Assorted improvements for docs

settings.LETTUCE_SERVE_ADMIN_MEDIA forces lettuce built-in server to deliver admin static files. closes #158

Docs: trimmed and compressed the background image from >1MB to 250KB. Still far too large, but better.

- Reduced JPEG quality from 85% to 30% which is still thoroughly high enough quality. This reduces it to about 500KB.
- Trimmed background image from almost 4000px wide to only 2000px (this will affect people with horizontal window resolution greater than 2000, but should the rest even have to download such a large image?)

Docs: reduce link target duplication in source code using Sphinx's rst_epilog feature to make some link targets available in all documents.

Also done a fair bit of making sentences start with a capital letter. But I haven't touched headings at present.

release 0.1.29, using multiprocessing instead of threading. It's faster, more reliable and has less tweaks