Add ValidateBasic to Tx interface (or just StdTx)

[cwgoes]

Proposed by @ebuchman - this can check positive gas, positive fee, not-too-many multisigs, etc. in a self-contained stateless abstraction.

[ebuchman]

It should be on the Tx interface, not just StdTx.

Basically any input coming from a user ever should have ValidateBasic method to sanity check the values in a stateless manner.

We already have it on Msg, but Txs can contain data beyond the Msgs, so tx.ValidateBasic should check those as well.

Unclear whether tx.ValidateBasic should run msg.ValidateBasic for all msgs - probably not, let the baseapp handle that for everyone.