handler/PUT: Handle signature verification through a custom reader. (#2066)

Change brings in a new signVerifyReader which provides a io.Reader
compatible reader, additionally implements Verify() function.

Verify() function validates the signature present in the incoming
request. This approach is choosen to avoid complexities involved
in using io.Pipe().

Thanks to Krishna for his inputs on this.

Fixes #2058
Fixes #2054
Fixes #2087

typed-errors.go

@@ -29,3 +29,6 @@ var errSignatureMismatch = errors.New("Signature does not match")
 
 // used when token used for authentication by the MinioBrowser has expired
 var errInvalidToken = errors.New("Invalid token")
+
+// If x-amz-content-sha256 header value mismatches with what we calculate.
+var errContentSHA256Mismatch = errors.New("sha256 mismatch")