npm child process tweaks

Basically, some detection for win32 to correct execFile behavior on that platform (npm.cmd vs npm). Also read a set of npm install flags from package.json (platform specific) if available, this is important especially if the 'master' process (that is installing primus-live as a package) was invoked using an npm installation flag. 
e.g. npm install --msvs_version=2013 
will tell node-gyp to use visual studio 2013 toolchain to compile platform specific binarys,  
Master process can now pass this flag(s) down into primus-live via its package.json via some means (see housemon patch for example using postinstall hook).
This will allow the current commit of housemon to install with little issue on windows 7/8. (32/64), as node-gyp can use vc10/12/13 compilers based on this switch for things like engine.io and serialport (primus uses engine.io).
--lightbulb