updated to compile on latest node version

hey guys... we have updated the library to compile on the latest version of node (using node-gyp). it should be backwards compatible with at least 0.8.x as well. a few things:

when I went to push the code to github, I had to do a bunch of strange merging -- I don't really know why. there was also some whitespace mixing funkiness in the the node_sigar.cc file, which I converted to tab format (I use 2-space), which could be one of the reasons why it thinks I basically rewrote the whole file.

I changed the API a bit as well and simplified some of the C++ with v8u (no performance degradation). I force the module to be invoked via the `init` method and moved some of the functions, which did not require an instance of sigar to be used, up to the main exports.

I intend to continue working on more documentation and expand the javascript API to make it easier to use. I want to also upload node based system monitor, but I need to work on other things now, so I leave you with what I have done so far.

cheers