Added #toBase64 and #fromDroppedFile:

- #toBase64 is for situations where you want to control what happens to the URL. Maybe you want to upload it, rather than put it straight into an image - or maybe you want to use it as an image in a CSS background. At any rate, the code is exactly the same for toPng, but I put a lot of it into the toBase64 function
- #fromDroppedFile is very similar to fromEvent, only it assumes that you already have the file extracted from the drop event. This is useful e.g. with the Dropzone JS library, which exposes the file in an 'addedFile' event.

A problem about character conversion of psd.js

This helps the test pass. I think there is a bug with pngjs where it's emitting end before it's actually done writing.