Small type in test database name

NOTE:
':memory;' should be ':memory:', I remove NAME as django will be
automatically use in-memory if there is no NAME specified.

[ci skip]
Add output for recommends_precompute command

NOTE:
Even through logger has logged most of things, I think it maybe good to have some
output for user when they run recommends_precompute command

[ci skip]
Add use_raw_id option for similarities and recommendations

1. Add use_raw_id option (default = False) to all storages
2. Add test for returned raw id list

[ci skip]
Code formart according to flake8

1. format code
[ci skip]
travis settings update

1. Change pip install order, should install django at last
2. Add flake8 to check code style
Clean settings.py format

accroding to flake8
Add documentation and small CI change

1. update AUTHORS
2. update documentation
3. change CI settings to show django version
Change requirements.txt

move flake8 to test-requirements.txt
add dateutil in requirements.txt

[ci skip]
Add content_type_id as suggested

1. update documentation to have list example
2. return content_type_id in the list, now is list of dictionary
3. autopep8 all python code
4. change travis settings
Fix flake8 error

1. The E126 never shown in my local env as I didn't upgrade pep8 which
is now 1.4.6
Merge remote-tracking branch 'wgx731/develop' into develop

Conflicts:
	.travis.yml
	recommends/listeners.py
	recommends/tasks.py
	recommends/tests/tests.py
	setup.py
Introduced ``RECOMMENDS_STORAGE_MONGODB_FSYNC`` setting.