Provide support for iter(PDF) interface

[timClicks]

We currently only send a whole PDF back as a list of strings. It would be nice to support something like:

with open('nice-info.pdf') as f:
    for page in slate.PDF(f):
        for line in page:
            ...

This would break backward compatibility though.. so perhaps this would be an okay compromise?

with open('nice-info.pdf') as f:
    for page in iter(slate.PDF(f)):
        for line in page:
        # page would now be a `Page` object that yields lines,
        # rather than a plain str
            ...

[cancan101]

I am not sure there is a difference between those two forms.
Perhaps a better compromise would be to have:

with open('nice-info.pdf') as f:
    for page in slate.PDF(f).iter_pages():
        for line in page: