Include atomic mass evaluation data file in source

Right now, if the prebuilt nuc_data.h5 is not fetched, nuc_data_make will try to grab mass.mas12 from the atomic mass data center (AMDC). This pull request changes that process --- rather than downloading mass.mas12, it is now included directly in pyne/dbgen/. For those who are installing on machines that are not connected to the internet (yes, they exist), this allows PyNE to be installed with complete atomic mass and atomic weight data without having to download anything.