[#630] "Internalise" the current piwik code

Since the piwik app is so small, it seems easiest to integrate the code
in the rsr templatetags and then do the modifications needed there.

This commit just moves the code from django-piwik into rsr.
[#630] Add _patch_setattr function to partner sites middleware

...to get thread local settings to actually work

Without it the assignment to a thread local setting wont happen.

See https://gist.github.com/802020/b638e41fa8852d308468c02d07dd3e1365c33def
[#630] Add piwik_id field to PartnerSite

And to the admin and a migration
[#630] Modify piwik_tags to look for an ID in the partner site object
for the current partner site.
Merge pull request #722 from akvo/feature/630_sites_piwik_tracking

Looks good, but partner sites are slow on Vagrant, need to be checked where that comes from (probably not from this PR)