Reorganizes a bit the code in on_read_headers_block() of EvdHttpConnection
Adds new method set_keep_alive() to EvdHttpConnection
Checks that thread is not null before calling join() in stop() of EvdPoll

This can happen when EvdSocket have been created but never used during
program execution.
doc: Removes unexistent evd-marshal.xml from included docs
build: Fixes path to library's .la file in tests and examples Makefile
build: Fixes evd-marshal rules to fix distcheck target
Replaces angle brackets by double quotes in all locally included headers