Particle Masses

Hi,
I really like this calculator and use it frequently.
For calculations in physics I often need particle masses, so I added all leptons, quarks and massive bosons that are included in the standard model to a new constants category.
Regarding hadrons I also added proton and neutron as these are needed quite often.

For units I chose decimal powers of Electron-Volts/c^2, because normally calculations in elementary particle physics are performed using these.
For e, p and n I also added sonstants using SI-units (kg here), because they are frequently used for "non-particle" calculations.

As source for the values I used the Particle Data Booklet (http://pdg.lbl.gov/) as mentioned in the code.

I don't know if you consider these additions useful. If so, feel free to merge these changes back into the project!

Regards,
Johannes