Add support for DeleteObjects

[iamed2]

Right now there is no support for deleting multiple objects at once using a single request (the DeleteObjects API) without manually writing XML (see https://github.com/JuliaCloud/AWS.jl#limitations).

One possible action would be to implement support for translating the Delete structure to XML in AWS.jl, like boto3 does.

Another easier action would be to write XML generation in this package and present a wrapper function for DeleteObjects (this issue). The function could accept an iterator of object keys and delete them 1000 at a time, and maybe have recursive or glob delete functionality.