use node to make sure coffee command works on all platforms

on install on windows I get 

httpster@0.3.1 postinstall C:\Users\Scott\AppData\Roaming\npm\node_modules\httpster
./node_modules/coffee-script/bin/coffee -o lib/ src/
'.' is not recognized as an internal or external command,
operable program or batch file.
npm ERR! httpster@0.3.1 postinstall: ./node_modules/coffee-script/bin/coffee -o lib/ src/
npm ERR! cmd "/c" "./node_modules/coffee-script/bin/coffee -o lib/ src/" failed with 1
npm ERR!
npm ERR! Failed at the httpster@0.3.1 postinstall script.


Made a small fix to address this