Makefile: Add an option to compile with coverage.

Now you must compile with make COVERAGE=1 to compile with coverage
information. There is another rule (called coverage) which compiles with
coverage options and then runs gcovr (if available).
Algo: Delete MultiGermlines

Germline stored in a MultiGermline are also deleted by the destructor
Makefile: Testing gcovr existence

gcovr must not be in the current directory (don't use test -x) but
installed on the system (use ''which'' to test that)
Coverage: gcovr behaviour has changed

Between version 2 and version 3 the -r option doesn't have the same behaviour.
Now we need to the cd in the directory we are interested in. Weird.
remove test_lazy_msa.cpp

Old code, see 1e6b2b9
.travis.yml: coveralls.io upload, initial configuration