Sessions: Add global scope support

Add the ability to have a persistent global scope for your compiled
files. If you have a set of variables and mixins, you can easily parse
them once, then process multiple less files in the context of those
mixins/variables.

Specifically this is enabled by the following workflow:

### Create Session

    POST /session
       -> Some less code
            * mixins, variable definitions, ...
       <- Token

### Use the Session

    POST /?session=token
       -> Some less code
           * Can use the mixins defined during the creation of
             the above session.
       <- Compiled CSS

    POST /?session=token
    ...