Use Decimal instead of float on returned values

This primarily affects avail.linodeplans' "PRICE" key, which
previously returned stuff like 29.949999999999999.  I don't know
if there are any other float values returned, but they haven't
been evaluated.

This may break downstream users who can't handle Decimal, i.e.
json.dumps.
Merge pull request #8 from tjfontaine/decimalized-floats

Use Decimal instead of float on returned values