Condition HTTP connector compilation

## Summary
Patch to condition HTTP connector compilation.
Compilation of HTTP connector is conditioned via CMake variable (HTTP_CONNECTOR).
Tests/examples depending on HTTP connector are build if and only if the associated connector is activated.

## Travis
I have updated Travis configuration file to test both CMake configuration.

Thanks again for your library.

PS: Sorry for all the errors I have done in my pull requests. I hope this one will be fine :smiley: