browserify: Move require for Interest and ContentObject lower in the file to avoid circularity.

Change-Id: I8bcaa085b789c209a2f734b98e2e1ad8d1c7e9e0
browserify: Add requires as needed for Node.js

Change-Id: Id6cad3aa9ad630806a082153a723679d1658e7a7
browserify: Include KeyManager and Witness before files that use it.

Change-Id: Ic1ed144cb34efa7116691845732e333f05e8397b