devs: fix Dynamics::init() with negative Time

We add a check in the vle::devs::Simulation::init(...) function to
ensure that only positive Time are authorized. (Closes #120)
vle: add CXX and CC flags for Clang

By default, only GNU GCC and G++ have -Wall and -Wextra option. We add
option for Clang too.
devs: improve the positive and negative infinity test

From the C99, POSIX.1, for the isinf() function: "the standards merely
say that the return value is  nonzero if and only if the argument has an
infinite value". So, we update the check to test if isinf() > 0 or
isinf() < 0 insteaf of == 1 or == -1.
utils: remove unused variables
test: hide declaration of get_temporary_path

The get_temporary_path() function only necessary when Boost < 1.45 is
installed (Closes #107).
devs: remove unused variable

The FinishView declares an unused variable m_last. We remove it.
vle: use class instead of struct with Pimpl

For the PIMPL idiom, in header file, we use "class Pimpl" and in the
sources files, we use struct Pimpl. We homogenize declaration.
vpz: fix an error message

When adding a model that already exists into the coupled model,
the error message now gives more detailed information on the concerned
models.