Implemented a new transaction queue. Transactions with a holes in their
nonce sequence are also not propagated over the network.
N: 0,1,2,5,6,7 = propagate 0..2 -- 5..N is kept in the tx pool
The transaction pool will now some easily be able to pre determine the
validity of a transaction by checking the following:
* Account existst
* gas limit higher than the instrinsic gas
* enough funds to pay upfront costs
* nonce check
* chain manager sets td on block + td output w/ String
* added tx pool tests for removing/adding/validating
* tx pool now uses a set for txs instead of list.List
