Charge state prediction

Compose a feature which would provide for a distribution of models indicative of the various charge states available, as well as their relative abundances.  

Thoughts:
[[Molecule: NH3+..., 0.5], [Molecule NH2..., 0.5]]

Provide a thresholding value (% error? or pH unit driven decision?)

``` Ruby
m = Molecule.new("NCCN")

mat = m.matches "N" 

atom = mat.first.first

atom.charge += 1
```

...
