Added ability to support remote URIs

[boralyl]

Including images in a template that are remote (we use amazon S3 for our static content) is supported in pisa by passing a uri. As mentioned in the commit comment, this could also be done with a regex, not sure what would be most efficient.

[maestrofjp]

+1 for getting this merged in as we use S3 as well. The startsWith is just fine because its very readable whereas regex not so much. I suspect that loading up a regex might be slightly slower, but it's probably not worth investigating. FYI, since Python 2.5 you can pass a tuple into startsWith:

elif uri.startswith(("http://", "https://", )):

[taplister]

@maestrofjp Thanks for the tip, wasn't aware of the string.startswith(tuple, ...)

[maestrofjp]

@taplister you're welcome! I only came across it myself a few weeks ago...

[chrisglass]

Merging, hope this helps.