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
...