settings: Parse the ADMINS from $DJANGO_ADMINS

This allows specifying the $DJANGO_ADMINS environment variable as if it
were the "To:" field of an email:

    export DJANGO_ADMINS="Joe Bloggs <joe@bloggs.com>, Jane Fonda <jane@fondas.cc>"
settings: Set a sensible default for SERVER_EMAIL
settings: Ensure GOOGLE_ANALYTICS_* available when compressing

Context processors are not available when compressing offline. This
makes sure that the offline compression and the online asset lookup
agree on the content of the compressed JavaScript, by ensuring that the
Google Analytics code is rendered the same in both cases.

See:

- https://twitter.com/jezdez/status/435768328305782784
- http://django-compressor.readthedocs.org/en/latest/settings/#django.conf.settings.COMPRESS_OFFLINE_CONTEXT
settings: Add support for setting custom storage domain

This will allow us to serve assets and media from a CloudFront
distribution, solely by setting the

    $DJANGO_AWS_S3_CUSTOM_DOMAIN

environment variable.

Closes #39.
Fix pep8 errors and warnings
Further pep8/flakes fixes
Run flake8 as part of the Travis build

In order to ensure code consistency and quality, run the full flake8
linter on the codebase. This includes everything from pep8, pyflakes and
the mccabe complexity tester.

I've put it in `script` rather than `before_script` because I'm a
bastard and think quality regressions should fail the build. Currently,
the codebase is clean.
Merge pull request #52 from okfn/pep8-pyflakes

Pep8 and pyflakes tests in Travis
Rename site_processor to site (for cleanliness)

No other context processor ends with _processor, so let's not do that
here.
Merge remote-tracking branch 'origin/master' into haystack
Fix pep8 errors
Fix flakes error