Flash messages signals

If we send a signal when a template is rendered why not when a message
is flashed? One real world use case is in tests, this signal should make
flash messages expectation easier to implement.
fix typo in quickstart guide
Added more information about app factories.
Merge pull request #693 from untitaker/factory_docs

Added more information about app factories.
Merge pull request #692 from mikegagnon/patch-1

fix typo in quickstart guide
Merge pull request #687 from poiati/flash-signal

Flash messages signals
Documented new signal message_flashed