new api method assignTestCaseExecutionTask (initial version) #26
examples added for new api method assignTestCaseExecutionTask #26
integrate v0.5.0 listKeyword service methods #25

Merge branch 'master' into tl-future

Conflicts:
	CHANGES.rst
	src/testlink/version.py
getLastExecutionResult - new optional argument 'options' #27
prepare release v0.5.1
prepare release v0.5.1