[#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)
#721 add release notes
[#721] release notes - minor spelling fixes
Merge pull request #724 from akvo/feature/721-releasenotes

#721 add release notes
[#725, #630] Fix breaking of partner sites

Work around the SQL query that hangs by assembling the list of countries
in python instead.

Use prefetch_related() to (hopefully) minimize the number of queries
needed.
[#630] Add piwik tracking code to parter sites widgets
Merge pull request #726 from akvo/feature/630_sites_piwik_tracking

Feature/630 sites piwik tracking
[#710] Merge branch 'feature/710_rain_iati_import' into release/candidate
[#710] Merge branch 'feature/710_rain_iati_import' into release/candidate