update passwords and schema for splited keys and create tx to multiple address through a json string

[velasquez]

This is an addition I did to bitgo-cli to allow change passwords and schema (m of n) of previosuly created splited keys. This is important to be able to handle grant/revoke access to any user-created key.
It only changes 1 key at a time to avoid mistakes. It reads it form a json generated split key file, then uses 'genSplitKey' which i modified to accept an argument 'key' in 'params' and makes the split with that key, if not 'key' 'param' is specified calling 'getSplitKey', a new key is generated.

$ bitgo help updatesplitkey
usage: bitgo updatesplitkey [-h] [-m M] [-n N] [-f FILE] [-k KEY]

Optional arguments:
-h, --help Show this help message and exit.
-m M new number of shares required to reconstruct a key
-n N new total number of shares per key
-f FILE, --file FILE the input file (JSON format)
-k KEY, --key KEY key index to update

And i added the command 'createtxfromjson' allows to create transacctions to multiple addresses using json notation.
This is the help options for this command:
$ bin/bitgo help createtxfromjson
usage: bitgo createtxfromjson [-h] [-j JSON] [-f FEE] [-c COMMENT] [-p PREFIX]
[-u]

Optional arguments:
-h, --help Show this help message and exit.
-j JSON, --json JSON json string {str addr: int value_in_satoshis, ...}
-f FEE, --fee FEE fee to pay for transaction
-c COMMENT, --comment COMMENT
optional private comment
-p PREFIX, --prefix PREFIX
output file prefix
-u, --unconfirmed allow spending unconfirmed external inputs

[velasquez]

This PR now includes another feature: 'createtxfromjson' which is explained in the README.md

Please let me know what you comsider should be completed/changed to merge this changes.