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.

It must be able to return the default port for a given protocol.
(i.e. http - 80, https - 443)

We must be sure that people won't change this code, because we need to
care about backward compatibilty.

MARKER is not required, it just polutes the code.

And should not prevent a bad usage of the library.

There is no time yet for such changes in libnetworkcache.

Sorry, it should not be commited in the previous commit.

In this case, super does not beahvior as expected, so it is better to
call the init method directly.

Actually, the NetworkcacheClient must be able to fetch the
certificate files under a HTTPS server.

So, these classes and methods are going to be used to setup the
environment to test this feature.

Once the tests are growing up, I decided to organize it in a clean way.

test_libnetworkcache  - tests to NetworkcacheClient class
test_signature        - tests to signature script
libnetworkcachemixin  - an util module with all the code which is
                        required to setup the test environment

NOTE: THERE IS NO CODE CHANGE.

This test must prove that when the signature_certificate_file_list
has a path to the file system and urls, the file system path has high
priority over the urls.

To download contents in a correct way, it must use the
libnetworkcache.

This file must provide all the utils function, to avoid duplicating
code.