* add a DL button + serve the signer certificate file (the one used to
timestamp)
* fix the verification instruction (add -untrusted tsa_cert.pem)
* few CSS tweaks to improve page layout
as many configuration paths can be relative to the execution working
directory, this directory is recorded at launch time, and the set_params
function does a chdir(<start work dir>); <load conf>; chdir("/") when
it's called.
* split the rfc3161 and the http part
* add a logger function
* add a debugging logger function for the requests
* add a specific handler for rfc3161 request