This patch and tests introduces a context for all callbacks currently
in node-sass. Right now, they run w/ an ambiguous context, making the
`this` keyword not helpful to importer developers (and eventually
custom function developers). This creates a context object, adds the
render options to said context object, and then uses that context for
importer, success, and error callbacks via `this.options`.

By using a context instead of just `.call(options...)` there is a hook
for later should we find additional environmental information is
valuable to importer / custom function developers such as the node
environment, node-sass version, etc.