move dependency injection into a separate file - removes circular dependency.

This makes it tidier so that we don't `require('crypto')` while we are currently creating `crypto`.
Once you merge this I'll merge https://github.com/dominictarr/crypto-browserify/pull/59/files but just point it at
`require('browserify-aes/inject')` instead of `require('browserify-aes').__browserify`