Add main field to package.json
Add main field to package.json

Since the main file is created by the generator it should be put into
the package.json.  Fixes #14

joeybaker/generator-iojs,MrDHat/generator-node-mocha,thehobbit85/generator-node,stevemao/generator-node,zckrs/generator-node,RobertJGabriel/generator-node,ksheedlo/generator-node-mocha-jscs,rhettl/generator-node,liuxiong332/generator-node-gulp-coffee,yeoman/generator-node,joeybaker/generator-iojs,morgante/generator-nodepack,youngmountain/generator-node-gulp,davewasmer/generator-node,amitport/generator-node,iamstarkov/generator-node,canercandan/generator-node