User defined function breaks easily

[hunse]

If I add a an origin, choose user-defined function, and specify just a function as the expression (e.g. "min"), it breaks Nengo. There should be more checking that the expression string is well-formed.

[tbekolay]

I'm fixing this as part of my user preferences work in issue #170, as I've been essentially re-implementing the configurable dialogs. Once that issue is closed, I'll let you know so you can test out the user defined functions.