npm: add node-gyp build support

npm: add node-gyp build support

This commit adds all the infrastructure necessary for doing node-gyp builds of
mraa. Note this will only build the mraa js bindings and will dirty your git
tree whilst doing it. It also assumes the user does not have SWIG installed on
their machines and so the swig wrapper is static. The target to use this
feature is 'npmpkg'.

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>