piranha.hpp: some internal doc updates.
mp_rational: remove a couple of unused typedefs from the test.
mp_integer: workaround for a nasty problem involving wide char types, numeric_cast and clang.
sphinx doc: minor tutorial update.
series: beautify and simplify some internal functions related to term insertion.
piranha: small piece of internal todo.
Merge branch 'development' into new_series_operators