use npmignore instead of a postinstall script

Hello @nicks, 

Please review the following commits I made in branch 'nick-npmignore'.

68d0be4083b257e013b44392c0fa8d520304e5f3 (2014-08-20 10:26:00 -0400)
use npmignore instead of a postinstall script
the postinstall script was causing problems, because it uses
dependencies not specified in package.json

R=@nicks