Add .npmignore to avoid path conflicts

[marcofugaro]

I tried using this package with create-react-app and it breaks the webpack compilation because it has issues with the CaseSensitivePathsPlugin

here is the issue:

[CaseSensitivePathsPlugin] 
Module not found:
`/Users/marco/Code/repo/node_modules/react-codemirror/src/codemirror.js`
does not match the corresponding path on disk `Codemirror.js`.

this issue does not happen if in the package is not present a src/Codemirror.js

This should also solve issue #108

[sapegin]

It’s better to use files field in package.json and explicitly specify files you want to publish.