[MRG] Huge speed improvement for trees with many branches

[ndawe]

Don't bother remembering and reverting to original branch statuses, and avoid the slow SetBranchStatus().

Tested on a tree with 2256 branches.

Before:

>>> min(timeit.Timer("rnp.tree2array(t, branches=branches)", setup=setup).repeat(10, 1))
14.4979131222

After:

>>> min(timeit.Timer("rnp.tree2array(t, branches=branches)", setup=setup).repeat(10, 1))
0.362231016159

That's 14.5 seconds reduced to 0.4 seconds...