[MRG] User-defined functions

[trevorstephens]

Fixes #16

Still to do:

• add tests for the new function module
• update docs & examples for new API
• add example of creating & using a custom function

Welcome any feedback you may have @ibell or others interested in this feature.

[coveralls]

Coverage decreased (-2.3%) to 96.283% when pulling a3d0f9f on user-defined-functions into cdfe6bf on master.

[coveralls]

Coverage decreased (-0.03%) to 98.584% when pulling ac6b602 on user-defined-functions into cdfe6bf on master.

[coveralls]

Coverage increased (+0.1%) to 98.761% when pulling 6019ad1 on user-defined-functions into cdfe6bf on master.

[coveralls]

Coverage increased (+0.1%) to 98.761% when pulling ed1dc91 on user-defined-functions into cdfe6bf on master.