Add support for coverage/coveralls.io in cmake project.

This adds support for http://coveralls.io/ to the cmake project. This can then be run via a new Travis job, which uploads json containing the coverage data to the website.

To use this, please login usin github at http://coveralls.io/ and enable the Jansson project. You can then get a nice percentage badge for code coverage after each Travis buid. Coveralls will also comment on pull request with coverage info.

To test and run it locally do:

```bash
$ mkdir build && cd build
$ cmake -DJANSSON_COVERALLS=ON -DCMAKE_BUILD_TYPE=Debug ..
$ cmake --build .                      # $ make
$ cmake --build . --target coveralls   # $ make coveralls
```

There is also another script that generates a local HTML page using lcov CodeCoverage.cmake which can be run using

```bash
$ make coverage
```

The required depdencies to run this are:
gcov
curl
lcov (is needed for the normal CodeCoverage script)