Datapoint

+ Raise an error if data for the requested location is not provided from the datapoint API.

+ Remove stray print statement

+ Explicitly state the use of semantic versioning in `README.md`.
+ Add `elements()` function to `Timestep`.
+ Remove night/day indication from weather codes which have them.
+ Change the logic used to calculate the closest timestep to a datetime. The closest timestep to the datetime is now used. Add a new function, `Forecast.at_datetime(target)` to do this. `Forecast.now()` has been changed to use this new function. The old behaviour is deprecated and available using `Forecast.now_old()`. `Forecast.future()` has been changed to use this new function. The old behaviour is deprecated and available using `Forecast.future_old()`.
+ Check if keys are returned from datapoint api in `Manager.py`. Do not attempt to read the values from the dict if they are not there.

+ Retry the connection to datapoint if it fails (up to 10 times).
+ Use versioneer to set version number from git tag.
+ Fix failure to return forecast at midnight.
+ Add changelog.

+ Check that data is provided in tests.
+ Set weather element of `timestep` to 'not reported' if data is not provided.
+ Update examples to use `get_nearest_forecast_site` function.
+ Rename `get_all_sites()` to `get_forecast_sites()` and `get_nearest_site()` to `get_nearest_forecast_site()`.
+ Limit observations to sites within 20 km of the nearest observation site.
+ Require that the nearest location is within 30 km of the requested location.
+ Show the available sites on maps in the documentation.
+ Use a haversine function to calculate the distance between coordinates.
+ Use setuptools in `setup.py`.
+ Fix bug where site attributes were assigned incorrectly.
+ Use sphinx to generate documentation.
+ Fix bug where longitude or latitude values of 0 returned false in `get_nearest_site()`.

+ Remove stray print statements.