Add home button attributes to entries and themings.
public root url redirects to home url of matching theming

* add theming_request_scope config option to make selection of theming
  by request configurable.
* by default select theming by cname
* redirect if matching theming has home_url
inputs for home_url attributes in account theming form

allow setting home_url and home_button_enabled_by_default attributes
for an account's default theming via account admin edit form.
edit home_url and home_button_enabled attribute via editor
rename navigation home button to top button
allow specifying theme options via Themes#register
add configurable home button to navigation
disable home_button_enabled_by_default check box

disable if theme does not support home button
add disable option for input views

add option to disable backbone input views inside the editor. allow
displaying different inline help for disabled inputs.
placeholder option for text input backbone views
pass theming attributes to editor
refine edit meta data view in editor

* split into tabs
* disable home button inputs depending on theme support
* display placeholder with theming home_url
initialize home_button_enabled attribute on draft creation

set to theming home_button_enabled_by_default
style home button in default theme
make home_button support the default for themes

rename home_button option to no_home_button.
use pretty_theming url for home button
uncheck home_button_enabled checkbox if not supported
hide home_url input if home button not enabled
change default of home_button_enabled_by_default to true
Merge pull request #98 from tf/home-url

Home URL