Travis configuration to build on OSX too

Configuration for Travis CI to build both Linux and OSX. OSX building enabled for repo by Travis people so we just need to update our configs.

Moved out bootstrapping dependencies into own files for platforms. Not complete yet, it's meant to first be set up for Travis and then we might extend that to cover more platforms too. This is a proof of concept that it works.

There might be some problems with OpenSSL on OSX so we might need to let brew install that too.

Another thing to check is regarding the gcc compiler on OSX.