Add proxy configuration for timezone update

### Problem:
I was need a feature for specifying proxy settings in connection that gets TZ updates inside TimeZoneRegistryImpl.

### What have been done:
I have modified the code with proxy settings specified in ical4j.properties.
Also I add connect and read timeouts for the connection.

#### Settings in jcal4j.properties:
**net.fortuna.ical4j.timezone.update.timeout.connect** - Connect timeout ( milliseconds, e.g. 10000 - 10s, unlimited by default )
**net.fortuna.ical4j.timezone.update.timeout.read** - Read timeout ( milliseconds, e.g. 10000 - 10s, unlimited by default )
**net.fortuna.ical4j.timezone.update.proxy.enabled** - Use proxy ( true / false )
**net.fortuna.ical4j.timezone.update.proxy.type** - Proxy type ( DIRECT / HTTP / SOCKS )
**net.fortuna.ical4j.timezone.update.proxy.host** - Proxy server host name ( e.g. proxy.myorg.com )
**net.fortuna.ical4j.timezone.update.proxy.port** - Proxy server port number ( e.g. 3128 )

If I forgot something or something like that, let me know.