Mixin global context asset when rendering on server

Add the concept of a global context asset. This is a special asset that
contains extensions to `HAML.globals`. This asset is mixed into the
javascript environment before compilation on the server - allowing us
to share custom `HAML.globals` on both client and server.

The path for this asset is configurable.
Ignore .ruby-version for those using rbenv
Monkey patch rails: bust server template cache in development

When the global_context_asset changes rails should recompile stale hamlc
templates. By default, rails only recompiles templates if they're
touched, which is problematic since it's the helper not the template that
changed.

This commit patches rails so that it automatically recompiles and touches
hamlc templates when the global_context_asset changes from underneath it.
Update CHANGELOG.
Add some words about the open commit bit.