tide_gauge app and raw-measurements endpoint

Providing endpoints like: `/1/tide-gauges/measurements/gladstone-1/`

This is used to store raw measurements from tide gauges. These can be
processed and converted into what we currently call Observations.

https://www.pivotaltracker.com/story/show/78441914 [Delivers #78441914]
Merge pull request #112 from sealevelresearch/measurements-endpoint

tide_gauge app and raw-measurements endpoint