Reuse the request.context

Cornice service accept a "factory" parameter and pass it to the view.
The factory build an object in the pyramid framework, so, cornice
did not need to build it again.
Furthermore, by this change, factory can now be a string exactly
like in pyramid.
fix #212, using zc.recipe.egg to play well with zc.buildout >= 2
#213, require colander>=1.0b1 for cornice[test]
doc-fix : use colander SchemaNode's 'missing' instead of 'required', #210
Document correct import location for CatchErrors
Merge pull request #216 from mozilla-services/doc-CatchErrors

Document correct import location for CatchErrors
Merge pull request #214 from joesteeve/fixes

Fixes for #212, #213, #210
Merge pull request #209 from mardiros/request_context

Reuse the request.context
ext/sphinxext: fix py3k compat
Merge pull request #217 from abeutot/master

ext/sphinxext: fix py3k compat