Build system

This consists of 3 small changes:

1.  one-line change ( isnan --> std::isnan ) to make compilation work with -std=c++11

2.  put an "#ifdef USE_MPI" around every #include of the harp headers, to optionally include the MPI header.

3.  Add some Makefiles that allow building and linking everything.  These support:
$>  export SPECEX_PREFIX=/where/to/install
$>  make
$>  make install
$>  make uninstall
$>  make clean

Building with MPI support still does not work, but I understand the remaining problem, and will come chat with you about it soon...