Added support for onion http c library

Hi,

I added a new test for the onion http c library. I had no time to properly check if it works on a virtual machine, but everything looks like it should.

Onion is not a proper framework, so it had to use all the c libraries: libjson, mysql-client. Also it auto downloads and compiles onion with the onion target at the makefile; it it needs to be automated, it can just be added to the "all" target.

If you have any problem or request, please don't hesitate to contact me.