Supported choosing asset_id to fee for simple transfer

[brickgao]

#22

[xeroc]

Thanks for the PR. I would like to make a request that is pretty easy to implement and enables all calls to use a different fee asset.

1. use fee_asset to be used in **kwargs of all methods (fee_asset should be of type bitshares.asset.Asset)
2. do not touch individual methods (like transfer) but instead modify finalizeOp (similar to append_to)
3. Modify transactionbuilder.py and create a new method set_fee_asset that sets the fee for the object
4. Modify constructTx and addRequeredFees to allow using a specified asset id.

That way, all calls can make use of this feature without even touching them.

[brickgao]

Hi @xeroc, thanks for your advice, and I have updated.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@c7f9899). Click here to learn what that means.
The diff coverage is 57.14%.

@@            Coverage Diff            @@
##             master      #56   +/-   ##
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  Coverage          ?   68.15%           
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  Files             ?       50           
  Lines             ?     3925           
  Branches          ?        0           
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  Hits              ?     2675           
  Misses            ?     1250           
  Partials          ?        0

Impacted Files	Coverage Δ
bitshares/bitshares.py	47.34% <50%> (ø)
bitshares/transactionbuilder.py	66.52% <75%> (ø)

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[xeroc]

Excellent work! Thank you for your contribution!