Merge pull request #2 from tclose/divided_up_user_layer

Refactored user_layer into separate modules
Merge pull request #6 from tclose/removed_e_from_builtins

removed 'e' from built-in constants (as it is sometimes used for reversal potential)
Merge remote-tracking branch 'andrew/modular' into removed_e_from_builtins
added more sophisticated '^' escaping to account for brackets
Merge branch 'modular' of https://github.com/apdavison/nineml
Merge branch 'removed_e_from_builtins'
fixed up regex for before '^' matching
Merge branch 'removed_e_from_builtins'