coveralls: send coverage report to coveralls.io

coveralls.io is a service that uses information from gcov (generated during
"make test") to provide a coverage report online:

    https://coveralls.io/r/01org/parameter-framework



Some files must be ignored, specifically: generated files (python bindings and
files generated by CMake), tests (because they would inflate the coverage rate)
and samples (for now, only the skeleton subsystem).

A "badge" is also added to the readme showing the coverage rate.

Signed-off-by: David Wagner <david.wagner@intel.com>