Extracts version from ECKey private/public key utility class

The version bytes are unrelated to the majority of all `ECKey` functionality; and should (IMHO) be unrelated to the private/public convenience utility/storage class that it represents.

Private/public keys are not attributable to a network, addresses and wallets (aka collections of addresses) are.
Therefore the abstraction of a network version should be moved there, instead of complicating what should be a simple convenience wrapper.

I have mutated the `ECKey` API to only use the `version` parameter where it is actually necessary (when interacting with WIF or Addresses).

That is:

`ECKey.toWif`
`ECKey.getBitcoinAddress`
`ECKey.toString`

and

`ECPubKey.toWif`
`ECPubKey.toString`

For `toString` behaviour, the default behaviour is to print the WIF string. 
This remains the same, however `mainnet` is the default output.

For this API to be sane, I would argue that the default `toString` behaviour should be simply `toHex`; to more clearly represent what this storage class is actually representing (private/public keys).
However, this will require changes to HDwallet tests to stop relying on the current `toString` behaviour.  So for the purposes of limited change, I'll wait to see what the response is to this request in general.

I have also renamed `getBitcoinAddress` to `getAddress`, as it could represent any valid network.

This change also simplifies the `ECKey` tests greatly, as now it is simply testing the functionality of `ECKey` itself, and then individually testing the different networks for valid address generation.