Added wrap_once action to LayoutSlice

Refactored `wrap` into `pre_map` and `wrap`. Cleaned code and comments
redone.
Added update_attributes and map to LayoutSlice

Added some tests. `update_attributes` can be a useful action for setting
attributes on different fields of a layout.
Removed docs folder from setup

This should fix issue #132
Implementing missing private operator methods

This should fix issue #114
CHANGELOG updated
Removing old filter names, deprecated
Fixing error logging on |as_crispy_field

Avoid hiding errors away that can cause production problems, this should
fix #135.
Adding greedy parameter to filter
flatatt - Switch to from django.form.utils to internal app .utils
Avoid side effects when manipulating context in layout objects

Added a `KeepContext` context manager, that rolls back changes done to
context by layout objects, to avoid side effects in other layout
objects.

This allows the cleanup of some code and simplifies writing safe layout
objects, like the one proposed in issue #136.
Fixing Python 2.5 compatibility
Merge branch 'dev' of git://github.com/rockingskier/django-crispy-forms into dev
Updated CHANGELOG and CONTRIBUTORS
Bug fix for pickling crispy forms by @powderflask

See issue #107 for details
Don't display legend if not provided
Use formset iterator instead of forms list attribute
Fixing bug with multiple forms
Update index.rst
Merge branch 'empty-legend' of git://github.com/borgstrom/django-crispy-forms into dev
Merge branch 'formset_iterator' of git://github.com/danxshap/django-crispy-forms into dev