[MRG] User-defined fitness measure

[trevorstephens]

Fixes #6

Still to do:

• integrate the new fitness measures into code base
• add tests for the new fitness module
• update docs & examples for new API
• add example of creating & using a custom fitness

This feature will have a similar user API as used in #18

[coveralls]

Coverage decreased (-0.9%) to 98.226% when pulling 73dbf08 on custom-fitness into 2b45637 on master.

[coveralls]

Coverage decreased (-0.5%) to 98.68% when pulling 7d0d10d on custom-fitness into 2b45637 on master.

[coveralls]

Coverage decreased (-0.1%) to 99.015% when pulling f185d47 on custom-fitness into 2b45637 on master.

[coveralls]

Coverage increased (+0.03%) to 99.179% when pulling 2d8a5b5 on custom-fitness into 2b45637 on master.

[coveralls]

Coverage increased (+0.03%) to 99.179% when pulling af06277 on custom-fitness into 2b45637 on master.