Qt Logging, waitSignals

• Messages sent by qDebug, qWarning, qCritical are captured and displayed when tests fail, similar to pytest-catchlog. Also, tests can be configured to automatically fail if an unexpected message is generated. (See docs).
• New method waitSignals: will block untill all signals given are triggered, see docs (thanks @The-Compiler for idea and complete PR).
• New parameter raising to waitSignals and waitSignals: when True (defaults to False) will raise a qtbot.SignalTimeoutError exception when timeout is reached, see docs (thanks again to @The-Compiler for idea and complete PR).
• pytest-qt now requires pytest version >= 2.7.

Internal changes to improve memory management

• QApplication.exit() is no longer called at the end of the test session and the QApplication instance is not garbage collected anymore;
• QtBot no longer receives a QApplication as a parameter in the constructor, always referencing QApplication.instance() now; this avoids keeping an extra reference in the qtbot instances.
• deleteLater is called on widgets added in QtBot.addWidget at the end of each test;
• QApplication.processEvents() is called at the end of each test to make sure widgets are cleaned up;