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.
Use `to_s` so asset AND it's dependencies are loaded into context

Asset#body does not include content injected with sprocket directives
Update specs to use GlobalContext#to_s
Merge pull request #90 from thedeeno/global_context

Use `Asset#to_s` so file AND dependencies are fetched from the pipeline
Load Hamlc helpers from GlobalContext

Instead of including hamlcoffee helpers manually for server side templates
(where we have to manage the compilation pipeline) just expect them to be
injected into the global context asset. Like so (if using the default,
context path):

# app/javascripts/templates/context.js.<handler>.<handler>...
//= require hamlcoffee

This simplifies the HamlCoffeeAssets module by allowing us to delete the
helper related code. All helpers come in through the global context asset
Monkey patch ActionView::Template so it doesn't swallow ExecJS exceptions
Update readme to reflect change to global context helper inclusion
HamlCoffeeAssets#helpers returns global context (for back compatibility)
Merge pull request #91 from thedeeno/asset-dependencies

Load Hamlc helpers from GlobalContext
Merge branch 'master' into rename_template_path_config
Use yard deprecation tag, fix typo