Scrapy-testmaster

Added an example project to more concretely demonstrate some of the affordances of this library. Updated the README to explain this example project, and also corrected an error in my description of how to produce a valid FormRequest using the REQUESTS_TO_ADD field in a local config file. Finally, cleaned up some ugly code in middleware.py, and made a small change in *utils.py* which will hopefully resolve the bug whereby it was impossible to update spiders with custom arguments that were edited within the spider's __init__ function.

Fixed bug whereby the `update` command was observing the max fixtures value in overwrites (i.e. refusing to write over a fixture if the of fixtures >= the max fixtures value).

Discovered an edge case where a problem was arising for the *parse* command. If you tried to trigger `testmaster parse` with depth > 1 and specifying a callback which outputs one or more requests which directly inherit the callback value from the initial request (i.e. request.callback = response.request.callback), then the callback value was being set to a function within *parse.py* (`prepare_request.callback`), rather than the appropriate function of the spider.

Because of some Iterator silliness, only the first custom validation method was being tried previously. Now resolved.