No support for private chains / testnets in (operations.py & commit.py)

[nnnarvaez]

operations.py https://github.com/steemit/steem-python/pull/282

The asset_precision DICT should take the chain and assets from /steembase/chains.py and not be statically linked to the asset names?

Left a comment in the file as i havent had the time to look deeper to perform an iteration and get the proper asset names.

Maybe is possible to use the NAI of the asset instead that remains the same across chains ?

This also happens in commit.py line 645 https://github.com/steemit/steem-python/blob/57fb0f1436fbea27a74386d35815f45fb3076814/steem/commit.py#L645