A couple of notes/todo.
A couple of comment style fixes.
real: introduce a workaround for a performance problem with MPFR's fma().
integer: fix a problem in clang regarding the access to a private class during
partial template specialization.

It seems a rather corner-case occurrence, this should be safer and looks better
in the doc too. Tested on Clang and GCC 4.8/4.9.
mp_integer: add todo note.
Merge branch 'master' into quadmath
bmp floats: same performance workaround as for piranha::real.