Added ultra-fast RELL bootstrapping that can be invoked via -f D as described in the following paper:

B.Q. Minh, M.A.T. Nguyen, and A. von Haeseler (2013)
Ultra-Fast Approximation for Phylogenetic Bootstrap.
Mol. Biol. Evol., 30, 1188-1195.

It does not work yet with the MPI, nor the hybrid MPI/PThreads version and is still
in the test phase!