Pyne2001

Changed
- Use the `FC` environment variable as the FORTRAN compiler, instead of enforcing `FC = gfortran` in `Makefile`
- Removed `-ffpe-summary=none` compilation option, to remain compatible with gfortran version <= 4

Added
- Input DM or distance now enforced to be > 1e-6, because NE2001 executable hangs on zero (or close to zero) input.
- More unit tests

Added
- Version number is now specified in a unique location (`_version.py`) and can be accessed by the user as `pyne2001.__version__`

Added
- Can now be installed easily via `pip`. Directory structure has been modified as a result.

Added
- First release of pyne2001