Changed failmessage when entering unkown command

New users should be aware of the existence
of the "help" command when they try the
pfw for their first time
Merge branch 'master' of https://github.com/01org/parameter-framework
Implemented -h command for test-platform

For new users, it was not very clear that you had
to give an .xml to the test-platform executable

This patch implements -h an explicits that
Merge pull request #4 from kir0gawa/help-messages

Help messages