Added Configuration class

Hello, so I went ahead and implemented a class to hold key/values read from a .properties style file. The application just calls `loadConfig()` once on startup and it will search in some default locations for `opentsdb.conf` to load settings from. Alternatively the user can submit the full path to a config file via a command line passed to `loadConfig(String file)`.

This commit only includes the class and doesn't modify OpenTSDB to use it. I'll work on modifying the main app to use this new config class. All CLI options will override whatever config files it finds. I'll also use the `Const` class for default values across the app. I'd like to get away from using the `System.setProperty()` method so I'd move those variables into this class.

Let me know what ya'll think, thanks!