install.sh: no longer run mpi_sequential regression tests
install.sh: get rid of .env script, do python setup.py develop instead
Add symlink to demo in regression test folder

This is needed for the regression tests to work properly if the PyOP2
root directory is not on the PYTHONPATH.
Add mpi4py dependency to setup.py
install.sh: don't explicitly install dependencies that are required by setup.py
Output sensible error message when neither Cython nor C sources available
Fall back to distutils setup & Extension if setuptools not available
Use setuptools setup_requires to install NumPy before running setup

Use a helper class NumpyExtension to encapsulate the import from
numpy and delay it until NumPy has been installed.

Caveat: if NumPy is not available, it will be installed even for
simple operations like --help or --version