Explicitly check if an object is a class is pydispatch.

* This is to fix a disagreement in between CPython and PyPy.
* https://gist.github.com/4220533
PEP8 on pydispatch/robustapply.py
from_crawler added to spiders
Updated documentation of spider arguments to include required super call.
Fixed ordering of super argument call.
Merge pull request #350 from dellis23/master

This pull request fixes #349
Merge pull request #205 from joehillen/master

PyPy support
Merge pull request #330 from nramirezuy/spider-from_crawler

use from_crawler method if available in spiders
it is not possible to enforce an upper limit when latency is out of control