Add src to .npmignore

[bjankord]

Description of Issue

A common pattern for npm modules is to add the lib(es5) directory to the .gitignore and add the src(es6) directory to the .npmignore. Babel is an example of this pattern that also uses lerna.

More information can be found in this article:
http://jamesknelson.com/writing-npm-packages-with-es6-using-the-babel-6-cli/

• Add a .npmignore file to each package that contains the src directory.

[Matt-Butler]

Blocked on cerner/terra-ui#98

[bjankord]

This is partially complete now with #295 resolved. We've talked about not adding lib to .gitignore as we use files in lib with terra-site to do some testing. I think its possible we could add lib to .gitignore, we'd just want to generate those files whenever we run tests that reference them.

I'm thinking we close this and open a new issue for discussing what we want to do with lib. @Matt-Butler what are your thoughts?

[Matt-Butler]

+1 to adding a new issue to add lib to the .gitignore

#316