  This paper has been withdrawn due to an error in the proof of the termination
of the algorithm. Moreover, some ideas of this paper are a joint work with John
Perry and it is inappropriate to mention only me as an author. John Perry was
in spite of the statement in the acknowledgment of previous versions not
proofreading this paper. Recently we have found a new solution for ensuring
F5's termination, which can be found here : arXiv:1006.0318.
