Some code restructuration to allow devs to easy customize parsers or add new ones without import / define things in their settings
add documentation for changes about parsers
correction of the example in the doc
Merge branch 'master' into pull_143
improved customizing markup parsers

 - class-based markup parsers
 - rename pybb.markup_engines to pybb.markup
 - moved all markup related function to pybb.markup module
 - use new string-based defaults for PYBB_MARKUP_ENGINES, PYBB_QUOTE_ENGINES and PYBB_BODY_CLEANERS
 -more markup related tests
 - remove global pybb.util module variables: MARKUP_ENGINES, QUOTE_ENGINES, SMILES_DICT
Merge branch 'master' into pull_143
improved class based markup engines

- engines splitted to separate modules to prevent requiring not used libraries (bbcode/markdown)
 - improved deprecation warnings
 - actualized docs
 - use global variable for caching markup and quote engines
fixed test requirements
improve BaseParser class to allow engines to define their own CSS, JS, HTML Markup... and prepare pybb to use only one setting for current PYBB_QUOTE_ENGINES and PYBB_MARKUP_ENGINES
add missing load url from future for django 1.4
splitted parser and widget classes, removed ugettext from error strings, docs improvements
fixed warning template
bumped version