Implementing Split Transaction

[Pongch]

https://github.com/omisego/plasma-cli/issues/42

use:

plasma_cli split --fromutxo=UTXO_position --fromowner="from_address" --privatekey="from_privatekey" --toowner="to_address" --toamount=to_amount  --outputs=number_of_outputs --watcher="watcher_url"

Takes in a single UTXO input - split into N Outputs

• if have change, add change into the next output
• throws error if:
  1. outputs are more than 4
  2. amount to split > from amount

Note: Split is reimplementing a lot of function found in Send right now. I will refactor all the PlasmaTransactions and have them share the same interface later.

[Pongch]

fixed https://github.com/omisego/plasma-cli/issues/41