Posynomials with arithmetic

```
In [1]: import gpkit
In [2]: x, y = gpkit.monify('x y')
In [3]: p = 3.14*x*y**2 + y/2 - x**3*6*y + 2
In [4]: p
Out[4]: 2 + 0.5*y + 3.14*y^2*x + -6*y*x^3
In [5]: p.monomials
Out[5]: {3.14*y^2*x, 2, 0.5*y, -6*y*x^3}
```

Seems to be working. Only lightly tested / covered, so another set of eyes would be appreciated.

Also includes / requires mosify (to make making lots of monomials easier) and the dictionary interface for making monomials (which it doesn't strictly need, but heck).