Merge PR #2376: auth: Don't recalculate mempool fees for every msg signer, misc. cleanup

This PR begins improving the godocs for the auth module, and begins cleaning
up the Ante handler.

Additionally we previously calculated if the fee was sufficient for the tx
on every single signer. This is now refactored to be more efficient, and have
a better logical flow. No changelog entry as this is new to this release.

cmd/gaia/app/benchmarks/txsize_test.go

@@ -26,7 +26,7 @@ func ExampleTxSendSize() {
 		Outputs: []bank.Output{bank.NewOutput(addr2, coins)},
 	}
 	sig, _ := priv1.Sign(msg1.GetSignBytes())
-	sigs := []auth.StdSignature{auth.StdSignature{nil, sig, 0, 0}}
+	sigs := []auth.StdSignature{{nil, sig, 0, 0}}
 	tx := auth.NewStdTx([]sdk.Msg{msg1}, auth.NewStdFee(0, coins...), sigs, "")
 	fmt.Println(len(cdc.MustMarshalBinaryBare([]sdk.Msg{msg1})))
 	fmt.Println(len(cdc.MustMarshalBinaryBare(tx)))