Implement some initial recipes

[ctb]

See e.g. http://davis-assembly-masterclass-2013.readthedocs.org/en/latest/titus-notes.html

[ctb]

Main question: where should these go? Into khmer-protocols, or khmer itself, or ...? I'm thinking khmer itself.

[ctb]

Also see #387.

[ctb]

@mr-c suggests they go into khmer as user documentation that is part of some automated tests.

@camillescott suggests IPython Notebooks which @ctb likes because there are output graphs.

@mr-c thinks IPython Notebook would be best when we have a stable Python API.

@qingpeng points out that ipynbs are not good for long-running things.

So, how about minimal extensions or scripting to auto-generate figures that are then included back in the reST document from the recipes?

[ctb]

This is done in the khmer-recipes project.