Implemented support for derived streams.

Derived streams are automatically populated by data from other
streams, postprocessed by an operator. They can currently be
used to compute a sum of multiple different streams or a derivative
of another stream.
Require operators to implement update method.
Made 'derived_from' and 'contributes_to' tags optional.
Removed empty method.
Changed the order of timestamp and value.
Changed the way user-provided parameters are validated.
Changed operator parameter passing.
Improved readability.
Fixed expression and inc should be set.
Added PEP8 na PyLint to Travis CI tests. Fixed code style.
Ignore star args PyLint warning.
Not using Django lint.
Merge github.com:wlanslovenija/datastream