UMD support

[kamilkisiela]

Fixes #3

Use /src as the root directory. No more /dist/src/index.js, just /dist/index.js.
Create /dist/bundle.umd.js
Use module, jsnext:main for /dist/index.js
Use main for /dist/bundle.umd.js

Fix shadow variable apolloFetch in /src/apollo-fetch.ts

[jbaxleyiii]

@kamilkisiela thank you! @evanshauser this looks great to me! I don't want it to conflict with your learna work though, do you want me to merge this or wait and let you handle it?

[evans]

@kamilkisiela Thank you so much for adding umd support! The repo is now converted over to lerna. I wasn't sure what the transition would look like, and now it seems that your changes would be almost drop into the apollo-fetch package! Could you rebase when you get a chance? Then I'll merge the changes in

[kamilkisiela]

@evanshauser @jbaxleyiii done

[evans]

@kamilkisiela Great work, thank you for adding UMD support!