Periodically import EA measurement data

- New ea_sea_levels Django app to record EA stations & measurements
- Scraper to download & import EA station measurements
- Run script for heroku's scheduler addon.
- Updated README to document how to install said addon.

Example EA station measurement page:
http://apps.environment-agency.gov.uk/river-and-sea-levels/120705.aspx?stationId=5084

https://www.pivotaltracker.com/story/show/75972824
[Delivers #75972824]