Add defaultProps and propTypes for each component

[christopherhafke]

This will improve the type safety and usability of each of our components.

[christopherhafke]

@alex-gedeon Maybe we can link this issue with #34. In principle, components should be able to mount without crashing in the unit tests without any explicitly declared props (this is a good test that the defaultProps are correct), so this may be a good time to add such renders without crashing unit tests (there may be some exceptions to this). This shouldn't be too much overhead for this patch, since the code will look the same for just about every component (see App.test.js for what these tests might look like).