make tests more chatty
switch settings_test db back to sqllite

working around content_types caching problem with tests and mysql
fix import error in rooibos.storage.functions

making import of rooibos.contrib.pyPdf.pdf explicit
back to mysql for tests

content_types caching problem is worked around by putting
'django.contrib.contenttypes', before 'django.contrib.auth' in
INSTALLED_APPS, which makes sense, except the ways in which it doesn't.
added mock to requirements

which seemed to make a ton of sense until I noticed
rooibos.solr.models.disconnect_signals -- but still, it seems like a
good thing to work into some tests.
added no_signals() to a bunch of storage.tests

Suddenly they almost all pass, except for two.  I'd like to figure out
a more elegant way to do this - i.e. a way that every function in a
class would automatically call no_signals() without it having to be
called. But this seems to work for now.  Oh, I also refactored
ProxyUrlTest.setup.user to proxytest_user, hoping it would fix an error
OperationalError: (1061, "Duplicate key name
'access_accesscontrol_846f0221'")
when the test runs, but alas.