Specify the 'main' property for the 'curl' package.

To allow one to specifiy curl as a dependency like any other AMD module (e.g. to create a run as an [AMD Module](https://github.com/cujojs/cram/blob/master/docs/launch.md#run-module-configuration "Using an AMD module to configure curl.js").

Otherwise `define(['curl'], function(curl) {...}` would attempt to load `lib\curl\src\curl\main.j` which doesn't exist...