Added Celery for background tasks.

The implementation is still rather simple, using hard-coded configuration
values. This will change in subsequent commits.

The worker can be started with "manage.py operations worker". Celery
Worker CLI options can be passed after a double dash, like this:

    ./manage.py operations worker -- -C -E

requirements.txt

@@ -7,6 +7,7 @@ bcrypt==3.1.3
 blinker==1.4
 bugsnag==2.3.1
 bleach==1.4.3
+celery[redis]==4.0.2
 commonmark==0.7.2
 Eve==0.7.3
 Flask==0.12.2
@@ -29,6 +30,8 @@ wheel==0.29.0
 zencoder==0.6.5
 
 # Secondary requirements
+amqp==2.1.4
+billiard==3.5.0.2
 Flask-PyMongo==0.4.1
 CommonMark==0.7.2
 cerberus==0.9.2
@@ -38,15 +41,18 @@ html5lib==0.9999999
 googleapis-common-protos==1.1.0
 itsdangerous==0.24
 jinja2==2.9.6
+kombu==4.0.2
 oauth2client==2.0.2
 oauthlib==2.0.1
 protobuf==3.0.0b2.post2
 protorpc==0.11.1
 pyasn1-modules==0.0.8
 pymongo==3.4.0
+pytz==2017.2
 requests-oauthlib==0.7.0
 rsa==3.4.2
 simplejson==3.10.0
 six==1.10.0
+vine==1.1.3
 WTForms==2.1
 werkzeug==0.11.15