Make port setting safer

Allow either an integer or string in the JSON config for port (needed for the Ant-style ${myport} config to work.
Removed the host/port validation from the OpenTSDBGenericWriter.java - validateSetup() is called after start() (so socket creation spits out NPE's) and some subclasses might not connect to a server (eg; a file writer)
Resolve hostname once per writer

Instead of for every metric line
Merge pull request #2 from hynd/master

Make port setting safer