API Support - Transaction pool methods

[JonathanOppenheimer]

These methods are implemented in PR #65 but have no tests.

Methods:

• GetPoolTransactions()
• GetPoolTransaction()
• GetPoolNonce()
• Stats()
• TxPoolContent()
• TxPoolContentFrom()
• SubscribeNewTxsEvent()

[JonathanOppenheimer]

#117: GetPoolTransaction()