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>

docs/npm.md

@@ -0,0 +1,27 @@
+mraa NPM pkg                        {#npmpkg}
+============
+
+What you are looking at is a tarball made after 'make npmpkg' has been run on
+our build system. It contains a few prebuilt files in order to build easily
+with node-gyp on most systems. Note that this is not the preffered way of
+building libmraa and if you'd like to contribute, please learn how the build
+system works and get your sources from git.
+
+Any patches for mraajsJAVASCRIPT_wrap.cxx or version.c will be ignored as these
+are automatically generated files. When you execute a build from node-gyp/npm
+you do not generate the mraajsJAVASCRIPT_wrap.cxx file but rather use one that
+we have generated for you.
+
+Recreating tarball
+------------------
+
+From a checkout of mraa you can do the following to 'generate' this tarball
+
+~~~~~~~~~~~~~{.sh}
+mkdir build
+cd build
+cmake -DBUILDSWIGNODE=ON  ..
+make npmpkg
+cd ../
+tar czv --exclude='build/*' --exclude='.git/*' -f /tmp/mraa.tar.gz .
+~~~~~~~~~~~~~