Fixes compatibility with React 15.4

#105

• It doesn't rely on internal modules or React, so future React versions should not break this module again.
• It removes shallowCompare() since it doesn't really do anything
• It removes support to style (inline styles and className) the main react-portal div. Use a wrapper div (or other element) instead. This is a braking change if you rely on this functionality; thus, major release.