Return first available task. Close #135
Return first available task. Close #135

This commit reduces the overhead of querying for all available tasks. The first time
that one task is ready for the user is sent.

OpenNewsLabs/pybossa,geotagx/geotagx-pybossa-archive,geotagx/geotagx-pybossa-archive,PyBossa/pybossa,CulturePlex/pybossa,proyectos-analizo-info/pybossa-analizo-info,Scifabric/pybossa,OpenNewsLabs/pybossa,Scifabric/pybossa,harihpr/tweetclickers,inteligencia-coletiva-lsd/pybossa,harihpr/tweetclickers,jean/pybossa,geotagx/geotagx-pybossa-archive,CulturePlex/pybossa,stefanhahmann/pybossa,inteligencia-coletiva-lsd/pybossa,proyectos-analizo-info/pybossa-analizo-info,CulturePlex/pybossa,stefanhahmann/pybossa,jean/pybossa,geotagx/geotagx-pybossa-archive,geotagx/geotagx-pybossa-archive,geotagx/pybossa,PyBossa/pybossa,proyectos-analizo-info/pybossa-analizo-info,geotagx/pybossa