Updated ErizoAPI with node-gyp support

Hi all,

As some of you might know, we still used node-waf for building erizoAPI as a node.js addon. Today we have added node-gyp support as node-waf is being abandoned by the node devs. In fact, in some repositories node-waf has already been removed.
For now we will continue supporting both but in the long term, node-waf support will be dropped.

Pedro Rodriguez