Add serveralias directive

Now it is possible to set alternate names for a virtual host. A server alias
may contain wildcards:
 - '*' matches any sequence of zero or more characters
 - '?' matches one character unless that character is a period ('.')

Here is an example:

  <server server.domain.com>
    serveralias = server server2.domain.com server2
    serveralias = *.server.domain.com *.server?.domain.com
    ...
  </server>
Add 'Last-Modified' header only for static files or 304 responses

It is a partial rollback of the commit 89f3ea22.
The browsers seem to use this header to do aggressive cache (using some
strange heuristics), without doing any request to the server. IMHO, it is
a bug in browsers. Nevertheless, this header is useless for dynamic resources.
So we can skip it. When this header is meaningful for user's application, it
could be added explicitly.
Add the global directive 'src_dir' to define source directories

Yaws will compile all erlang modules found in these directories. The
compilation occurs when the configuration is loaded or reloaded. The
include_dir directives are used to search for include files. Multiple
src_dir directives may be used. There is no such directory configured by
default.