use new dollar units

[bqpd]

See https://github.com/convexopt/gpkit/pull/490

[whoburg]

This looks great. @bqpd points out we might want to define dollars relative to a specific year (to deal with inflation), but this PR is already strictly better than using counts, so I'm ready to merge it.

[whoburg]

Actually hold up -- this doesn't run -- we might need to update the gpkit install script to copy gpkit_units.txt and gpkit_constants.txt to the target folder?

Traceback (most recent call last):
  File "cost.py", line 2, in <module>
    from gpkit.shortcuts import Var, Model
  File "/Users/whoburg/anaconda/envs/gpkit/lib/python2.7/site-packages/gpkit/__init__.py", line 84, in <module>
    enable_units()
  File "/Users/whoburg/anaconda/envs/gpkit/lib/python2.7/site-packages/gpkit/__init__.py", line 40, in enable_units
    UNIT_REGISTRY = pint.UnitRegistry(path)
  File "/Users/whoburg/anaconda/envs/gpkit/lib/python2.7/site-packages/pint/unit.py", line 526, in __init__
    self.load_definitions(filename)
  File "/Users/whoburg/anaconda/envs/gpkit/lib/python2.7/site-packages/pint/unit.py", line 768, in load_definitions
    raise ValueError('While opening {0}\n{1}'.format(file, msg))
ValueError: While opening /Users/whoburg/anaconda/envs/gpkit/lib/python2.7/site-packages/gpkit/gpkit_units.txt
[Errno 2] No such file or directory: '/Users/whoburg/anaconda/envs/gpkit/lib/python2.7/site-packages/gpkit/gpkit_units.txt'

[bqpd]

Uh, it looks like it's finding a pip-installed version of gpkit, not the updated one with new units.

[whoburg]

I ran python setup.py install. It looks like the units txt files didn't get copied by the install script -- do they need to be added to MANIFEST?

[bqpd]

Oh, yeah, if you're installing like that they do.

[bqpd]

(general practice has been to only update the manifest on each pypi release, and otherwise install in pip by cloning github and then running pip install -e gpkit, which doesn't require an updated MANIFEST)

[whoburg]

ok, that worked. let's merge this.