Support for file upload

[afresh1]

We added this back in 0.15 and then got distracted and didn't create a PR for you. Have now ported those changes forward and gotten the PR out.

This should close #20

[jhthorsen]

Thanks for the PR, but it's very hard to evaluate because it combines a lot of different changes:

1. Image (non-text) upload (Not sure if I want to support this)
2. Change from a simple file to a meta and content file
3. Looks like it breaks _save_p() from other custom backends
4. Maybe more things I missed...

The title of this PR is also confusing, since mojopaste already supports uploads...

[afresh1]

I've been trying to figure out how to move forward on this, but need some additional guidance. The biggest is that you say it already supports file uploads, but I am not quite sure I follow how, could you expand on that?

If I tidy up the first commit and remove the other two, would that be better for review and more likely to be accepted? It seems that less than 0.15% of our internal pastes are non-text, so I think we would be OK with losing that feature.

[jhthorsen]

Support for uploads

I've totally missed out on what you meant here. You're absolutely right that Mojopaste does not support file uploads. I think the way I would solve this is to add drag/drop support with JavaScript. The dropped file would then be displayed in the text area after dropped. The help text has too be adjusted as well, or maybe add a background image.

Download filename

I'll take 2d93561 if you open a PR for just that.

Non-text uploads

I'm still unsure about this one. I might change my mind though...