Add preferred phase to Well class.

This entails:
 - Adding a new data member of type Opm::Phase::PhaseEnum.
 - Adding method getPreferredPhase().
 - Adding extra constructor argument.
 - Modifying constructor call in Schedule::addWell().
 - Adding unit test for the new preferredPhase property.
 - Modifying constructor call in all the well unit tests.
Merge pull request #172 from atgeirr/add-preferred-phase-to-wells

Add preferred phase to Well class.