Add "main" field to package.json

This allows users of bundlers like browserify and webpack to simply
`require('pep.js')` to get the `dist` build after installing PEP from npm.

Closes gh-183

package.json

@@ -1,6 +1,7 @@
 {
   "name": "pepjs",
   "version": "0.3.1-pre",
+  "main": "dist/pep.js",
   "description": "Polyfill of the PointerEvents W3C spec",
   "author": {
     "name": "jQuery Foundation and other contributors",