Refactor test_sea_levels

- Factor out boilerplate get/decode stuff
- Split TestSeaLevelsViewShowsObservations, only test 1 thing per test
Add predicted_{surge,sea}_level to /sea-levels/

https://www.pivotaltracker.com/story/show/73880096 [Delivers #73880096]
Merge pull request #84 from sealevelresearch/add-surge-to-sea-levels-endpoint

Add predicted_{surge,sea}_level to /sea-levels/