Future imports go after the docstrings

A `from __future__ import ...` statement must go after any docstrings;
since putting them before the docstring means the docstring loses its
magic and just becomes a string literal.  That then causes a syntax
error if there are further future statements after the false docstring.

This fixes issue #203, using the patch provided by @Arfrever.
Use lead blockprocessor comment as docstring

The lead comment in blockprocessors.py should really be a docstring for
consistency with other files.  Fix this with the other part of
@Arfrever's patch for issue #203.
Merge pull request #205 from Gamma3000/future-order

Future statement ordering