Fixed servlet for extra directories

Extra directories specified via the `-d` CLI option were not used when
running ri in `--server` mode.

The available extra documentation directories are displayed on the root
page and their content is accessible under URLs like `extra-1/`,
`extra-2/` and so on.

Conflicts:
	lib/rdoc/servlet.rb
Using store.titel as first choice for naming extra documentation directories

Conflicts:
	lib/rdoc/servlet.rb
Added tests for the extra directories implementation for the RDoc servlet

Conflicts:
	test/test_rdoc_servlet.rb
Update history for #173