Provide a convenience method to parse a proxy with format localhost:port

This issue relates to HTTPCORE-365 (which was the original issue about this topic). What I actually want to achieve is to have a convenience method to allow us to call the viaProxy() method with a String as parameter. E.g.:
Request.Get(uri).viaProxy("localhost:8080").execute();

Notes about the attached patch:

• I did not know where to put the new method "protected static HttpHost parseProxy(String proxy)" so I put it inside Request.java
• the parseProxy(String) is static because it is a utility class, so you can move to a more "appropriate place" and anyone can use it for other use-cases.
• following the suggestion of Gary Gregory, I made it compatible with IPv6 (following links below):
  • http://en.wikipedia.org/wiki/IPv6_address#Literal_IPv6_addresses_in_network_resource_identifiers
  • http://stackoverflow.com/questions/186829/ipv6-and-ports
  • http://serverfault.com/questions/205793/how-can-one-distinguish-the-host-and-the-port-in-an-ipv6-url

Hope to see it in 4.3.4 .