Added testpoolswap rpc method and python rpc test

[DeFiDev]

Added RPC method testpoolswap:

testpoolswap ( {"from":"str","tokenFrom":"str","amountFrom":n,"to":"str","tokenTo":"str","maxPrice":n} )

Tests a poolswap transaction with given metadata and returns poolswap result.

Arguments:
1. metadata                   (json object)
     {
       "from": "str",         (string, required) Address of the owner of tokenA.
       "tokenFrom": "str",    (string, required) One of the keys may be specified (id/symbol)
       "amountFrom": n,       (numeric, required) tokenFrom coins amount
       "to": "str",           (string, required) Address of the owner of tokenB.
       "tokenTo": "str",      (string, required) One of the keys may be specified (id/symbol)
       "maxPrice": n,         (numeric) Maximum acceptable price
     }

Result:
"amount@tokenId"    (string) The string with amount result of poolswap in format AMOUNT@TOKENID.

Throws RPC exception if input params were not valid.