Bump jclouds version to 1.5.7.

There is a bug in the old 1.5.0 version of jclouds that was used that
breaks compatibility with rackspace cloud files. Bump the version to be
compatible with rackspace cloud files.
Add dest path option to Blobstore.

Add a destination path option to Blobstore entries. This allows Jenkins
users to organize blobs in a hierarchy under the destination container.
Related to this do Jenkins build env variable interpolation in the
container and path fields.

Finally, allow for the path of a file relative to its workspace to be
appended to the path specified on a per project basis.

These changes make the jclouds blobstore an acceptable substitute for
the Jenkins SCP and FTP plugin.
Fix BlobStore jelly configs.

The BlobStore jelly configs were not able to properly save the
keepHierarchy boolean value. Move BlobStoreEntry jelly config into its
own file and use DataBoundConstructors to pass the field data in.
Update README with info on new blobstore options.

Add path and keep hierarchy info to the README Blobstore section.
Merge pull request #37 from cboylan/path-interpolation

Path specification and variable interpolation