Support custom celeryconfig modules

[phrfpeixoto]

I wanted to use this but had a custom celery config module, so I made this change to support it.

[coveralls]

Coverage remained the same at 95.031% when pulling 9f93d2c on phrfpeixoto:custom_celeryconfig into cf8aa80 on sontek:master.

[sontek]

@phrfpeixoto Could you implement this a little differently:

• Keep use_celeryconfig as a boolean.
• Introduce a new field celery_config_module (or a better name if you have one) and load it from there if set

So use_celeryconfig = true + celery_config_module = app.celeryconfig would do what you want, otherwise without a module said it would use the default behavior

[phrfpeixoto]

Will do.

[phrfpeixoto]

@sontek Done.

[phrfpeixoto]

Is this still relevant?