[index] finish keyvimerger integration as part of the python package

[hendrikmuhs]

with this PR the python package is fully self-contained again. keyvimerger already shipped with the package but the code in the C++ part was missing.

[hendrikmuhs]

removing make install broke C++ unit tests. But I consider this a good thing. Tests should run without running make install first, i somehow need to inject the keyvimerger binary location like for the python tests.

[coveralls]

Coverage increased (+0.02%) to 93.186% when pulling 0df5f31 on hendrikmuhs:index-keyvimerger-python into d308d2e on KeyviDev:index.

[hendrikmuhs]

to be reviewed as part of #37