Urlize mode bug fixes

Created by: dverdin

Fixing bugs in urlize mode :

1- message text was treated as attachment and remained on the server as either "msg.0.bin" or "msg.0.txt" files. Fixed by testing the part type before urlizing it. 2- some special characters in message id led to the attachment to be considered missing by Sympa.

I also introduce a test file for Message.t, testing only urlization. this test is set in travis and the test succeeds. Note that commits 065312d and fcadbac are related to travis. The first one to enable the test, the second one to have Sympa::Constants without running make.