[FEATURE] New command rackspacefiles:flushcontainer

This adds a new command and API method for flushing a whole container.
[FEATURE] Provide ability to specify additional headers for object

This features allows to provide additional headers (for example
"Content-Disposition") when creating a new container object. These
headers are sent when the respective object is accessed through
CloudFiles. Refer to the CloudFiles developer documentation for more
info on this.
[TASK] Optimize setting the meta data key
[WIP][FEATURE] Publishing Target for CloudFiles

This adds a very rough version of a publishing target which can replace
Flow's built-in publishing target. By default it publishes persistent
resources to the cloud. Static resources are not yet supported.

The target container is not yet configurable and defaults to
"typo3-flow-persistent-resources".