Add RFC3161 Timestamp Server Support

[daknob]

Currently eldim relies on the backend, be it OpenStack Swift or other, to accurately timestamp all uploaded files. However, for more visibility and for even more solid proofs that each file was uploaded at a particular time, eldim can make use of RFC3161 time servers.

These are publicly operated and trusted servers that sign messages given a particular hash of some content, adding a timestamp as well, to prove that this file existed at least as back as the date of the timestamp added by this server.

The configuration file should support multiple Timestamp Servers, each having a name and an address to contact. After obtaining the proper signatures, eldim should store all files obtained in the backends, using the $filename.$tssname.tsr file name.

A free timestamp server can be found here, operated by Free TSA.