This commit removes a lot of code from ghost.js, including:

Move helper functions registerThemeHelper and registerAsyncThemeHelper
to the helpers module.
Also update the app proxy object to reflect this new code location,
and the tests to reflect that as well

Create ./sore/server/filters which houses all filter related behavior.
Was previously on the ghost singleton.
Also create the filters_spec file for testing
and update all code and tests to use new code location.

Create ./sore/server/helpers/template which houses all template related behavior.
Was previously on the ghost singleton.
Also create the helpers_template_spec file for testing
and update all code and tests to use new code location.

Move ghost.mail instance onto the mail module directly
and update related code and tests to use new location

Move Polyglot instance onto require module directly

Move ghost.availablePlugins to plugins module directly
Merge pull request #1580 from hswolff/death-to-ghost-singleton

Move further away from the ghost singleton object
Updating to the latest casper

issue TryGhost/Ghost#1577

- adding asset helper for subdirectory support