It is not recommanded to use httplib directly, we should better
use urllib2, then this library will use httplib. This solve
issues with libnetworkcache when we try to use it behind an
http proxy.

We should not send the signature in the property dict. Otherwise,
the validation of json schema will fail.

Also fix formatting a bit.

It should return the file descriptor instead of file content as string.

This implementation does not consume RAM Memory, so it is safer to
download big files.

When you call file_descriptor.close, it will remove the file from tmp
folder automatically.

There is no need to read and store downloaded file. Returning the location is
good enough.

Rest of file requires hexdigested file.

There is no need to read whole file into memory on library level. So calculate
sum in chunks and give request method file descriptor.

Indeed, some day documentation shall be written, as soon as API will be defined.

HTTPConnection accept port as none, there is no need to play with it.

The select method  must return the file from shacache.
And the download method should return the file content instead of path.

If the directory has more than one entry, the lib must raise such Error.