Chaining unconfirmed transactions does not work

[merc1er]

Example:

from bitcash import Key

key = Key("YOUR KEY HERE")
key.get_balance()

key.send([], message="1")
# 0c849243fc76a9d1bef526e5d8c85f18382a83b1014eb4c363ec1726685cd5fd <- this transaction is valid
key.get_balance()
key.send([], message="2")
# 424bbe29c6f4ef4d4e7440e264555c5be6870e050155378a2971fe7fe875ae83 <- this transaction is not found in the explorer

Another example:

Here the balance does not make sense, since it would mean the second transaction is free (which is not the case).

[merc1er]

Fixed by #141