Compat: Put key as first argument.

Spec: In case of passing key (and updating shadir) require urlmd5 too.

Key is given by the library user. It has no internal meaning.

Note: Soon the best *one* module will be selected.

Required parameters are:

 * file: the name of the file
 * urlmd5: mdsum of orginal url
 * sha512: the hash (sha512) of the file content

Optional ones:

 * valid-until: the date which the file must be expired
 * architecture: computer architecture

And use this hash to check if upload was successful or not.

In order to create subresource of /path/ it is required to use POST not PUT.

References:

 * http://www.w3.org/Protocols/rfc2616/rfc2616-sec9.html#sec9.5
 * http://www.w3.org/Protocols/rfc2616/rfc2616-sec9.html#sec9.6

PUT to shacache shall be done to shacache path, not shacache path+sha sum.

This break backward compatibility during upload, but download is used more
often.