Refactored standard expression engine.
  - Established new parsing + execution sequence, easier to extend and debug.
  - Generalized tokens in order to support generic (no boolean, null or numeric) ones.
  - Fixed small parsing issues.
  - Added support for parameters in fragment selections (needed for #181).
  - Added support for expressions both in template name and in fragment spec in fragment selections.
  - Added support for expressions in variable names in assignation sequences (e.g. URL parameters).
  - Applied #186 and #188 to 2.1 development branch.