Issues 736 737 export error

Closes #736 
Closes #737 

This error was caused by the SQLAlchemy db instance not having a reference to the current app. Notice that it was raised only in functions that "yield" instead of "return", like in https://github.com/PyBossa/pybossa/blob/95f60cf27f473ab9a7607334b7929d13ebee0c73/pybossa/view/admin.py#L188-L192.

The funny think is that it did not arise in the tests, probably because of the test_context() used in them.

More information about it:
http://librelist.com/browser/flask/2010/8/30/sqlalchemy-init-app-problem/#b1c3beb68573efef4d6e571ebc68fa0b
http://stackoverflow.com/questions/19437883/when-scattering-flask-models-runtimeerror-application-not-registered-on-db-w
And specially here:
http://piotr.banaszkiewicz.org/blog/2012/06/29/flask-sqlalchemy-init_app/ where it is said that the approach taken in this PR might mess up db internals, while I don't know how true this assertion is, as the author seems not to know it either.

So, to avoid this, a different workaround to fix this issue can be taken, which is basically not using the "yield" in the functions and use "return" instead, but I think this same issue will arise again, sooner or later, so I prefer as proposed here.