added $field magic, don't fail when not enough arguments were given to a
magic, added tests
added setting for allow to override magic base setting per spider
allow to extract and assign only a part of the value generated by magic
field
remove the mention to $url magic: was not implemented, and anyway seems
an unnecesary shortcut (can be accessed via $response:url or $item:url)
Merge pull request #18 from andresp99999/hcf-middleware

After talking with Pablo I am making a first merge of this middleware, it is an initial version that still needs some work, but it can be used. I made all the changes suggested by Nicolas but two:

  * Adding the policies to define the slot handling strategy, this is nice to have but not strictly necessary to make it work, I am planning to add them soon.
  * The error handling, after talking with Pablo there is another approach we can take (which is having smaller jobs and using the closing reason to determine whether a job finished correctly or not). We may need to do some changes in the HCF backend to fully support this.