File upload functionality (80% done)

[ckrybus]

• [x] fix FileUploadInput widget rendering, fixed in #3
• [ ] add doc hint: need to use FileUploadInput widget explicitly
• [x] add docs: usage with javascript
• [ ] add tests

LATER:

• [ ] use FileUploadInput widget by default when rendering FileField/ImageField (inside field.html)

MAYBE:

• [ ] create custom FileField class?