Update README to more explicitly mention necessary to_s and email methods on User model

Fixes #436
Use Nokogiri 1.5.9 on Ruby versions < 1.9.2
Support configurable Forem.base_path

For example, in the case where I mount under my engine instead of the main application
the previous code wasn't working and I had to use:

Forem.base_path = MyExtension::Engine.routes.named_routes[:forem].path
Allow user class decoration to be invoked outside of the decorator.
Specify branch name for forem-theme-base so that bundle config local.forem-theme-base overrides work
Move factories to lib dir so that they can be required by the application for testing purposes
check forum read permissions when user replies to topic/post

Fixes #445
Fixes #446
Mention rails4 support in the README
mention necessary gemfile line for friendly_id for Rails 4
Specify cancan dependency also in readme
Switch to using Bootstrap CSS

Conflicts:
	app/views/forem/admin/categories/index.html.erb
	app/views/forem/admin/forums/index.html.erb
	app/views/forem/posts/_post.html.erb
	app/views/forem/topics/show.html.erb
Add btn-primary classes to categories form button
Fix category edit title
Change theme image
Correct README line for theme
Remove jquery.autocomplete in favour of select2

Conflicts:
	forem.gemspec
Improve admin navigation
Fix tests broken due to recent Bootstrap changes
No more Ruby 1.8 support for you.
Fix MySQL build failure caused by posts created too closely together (forum_spec)