iam: added ListAccessKeys and DeleteAccessKey

Also included integration tests for CreateAccessKey and these two new
functions.
iam: rename ListAccessKeys to AccessKeys

Also make UserName optional on AccessKeys and DeleteAccessKey methods.
iam: added group related methods (Create, List and Delete)
iam/iamtest: improve error message for invalid action
iam: added integration tests for group methods
iam: added UserPolicy related methods

Now it's possible to Put, Delete and Retrieve a user policy.
iam: update UserPolicy struct and PutUserPolicy method
iam: fix parameter ordering in GetUserPolicy

Making it consistent with PutUserPolicy.
iam: fix parameter ordering in DeleteUserPolicy
iam: added integration tests for user policy methods
iam: setting SecretKey to "secret" on createAccessKey
s3: add support for multipart uploads
Covering reviews.
Fixed PutPart retrying.
Merge from trunk.
Revert "fix".
Further hardening of the existing logic.
s3: add support for multipart uploads

R=jameinel, rog
CC=
https://codereview.appspot.com/7237068
s3: trivial fixes on tests

The last minute "obvious" changes went into the review,
but didn't get run.
s3: trivial fixes on tests

The last minute "obvious" changes went into the review,
but didn't get run.

R=
CC=
https://codereview.appspot.com/7300090