Specify ambiguity in eth_getTransactionCount

[Smithgift]

The JSON-RPC method eth_getTransactionCount, according to the specification, returns the number of transactions sent from an account. All well and good. In the majority of situations, this is the same as returning the nonce.

However, if the nonce does not start at 0, this leads to inconsistent behavior across clients.

a) In geth and Parity, getTransactionCount returns the nonce.
b) In pyethapp, getTransactionCount actually calculates the number of transactions, and returns that.

While this is irrelevant for the mainnet, it does matter for a testnet with multiple clients, or if an alternate nonce scheme was implemented. As written, it seems it should be pyethapp's behavior that is correct, but de facto it works as geth and Parity implement it.

I see three options:

1. Explicitly specify that getTransactionCount returns the transaction count, not the nonce.
2. Explicitly specify that getTransactionCount returns the nonce.
   2a) And maybe change the name to getNonce, too.
3. Specify that getTransactionCount returns the transaction count, but create a new method getNonce which returns the nonce.

References:

The pull request wherein I discovered this issue.
A short discussion in pyethapp's repo.

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