Added ability to specify hostname to bind to.

For example:

java NanoHTTPD -d wwwroot1 -h 127.0.0.1
java NanoHTTPD -d wwwroot2 -h 127.0.0.2
java NanoHTTPD -d wwwroot3 -h 127.0.0.3

And hosts file:

127.0.0.1   www1.example.com
127.0.0.2   www2.example.com
127.0.0.3   www3.example.com