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.

docs/sdk/core/app3.md

@@ -133,7 +133,7 @@ Now that we have a native model for accounts, it's time to introduce the native
 
 ```go
 // StdTx is a standard way to wrap a Msg with Fee and Signatures.
-// NOTE: the first signature is the FeePayer (Signatures must not be nil).
+// NOTE: the first signature is the fee payer (Signatures must not be nil).
 type StdTx struct {
 	Msgs       []sdk.Msg      `json:"msg"`
 	Fee        StdFee         `json:"fee"`
@@ -259,8 +259,7 @@ the same message could be executed over and over again.
 The PubKey is required for signature verification, but it is only required in
 the StdSignature once. From that point on, it will be stored in the account.
 
-The fee is paid by the first address returned by `msg.GetSigners()` for the first `Msg`, 
-as provided by the `FeePayer(tx Tx) sdk.AccAddress` function.
+The fee is paid by the first address returned by `msg.GetSigners()` for the first `Msg`.
 
 ## CoinKeeper