added prototype EventEmittingStateMachine

added a depedency towards the "evenement/evenement" library and
implemented the EventEmitterInterface inside the
EventEmittingStateMachine.
the following events are emitted by the state machine during execution:
* ON_EXECUTION_STARTED
* ON_STATE_ENTERED
* ON_STATE_EXITED
* ON_EXECUTION_SUSPENDED
* ON_EXECUTION_RESUMED
* ON_EXECUTION_FINISHED

refs #25 #26
state machine impl is now configurable within xml

needed to add support for a class attribute on the state machine, so the
schema version was bumped.

refs #25 #26
updated composers deps (removed patch version)
added code climate badge :D
execution context current state defaults to null

also bumped composer deps, refs #27
StateMachine.execute event is now optional,#28 #29
Merge branch 'observable-statemachine-26'

Conflicts:
	composer.lock
api doc + code cleaning
removed unused use statement.
adjusted chglog and readme for 0.5.0 release
last changes to readme+chglog before releasing
fixed psr2 violations