Add forPath method

[SentryMan]

Now can do stuff like this.

//given properties like 
database.example.password
database.example.username

Configuration dbConfiguration = Config.forPath("database");
Configuration dbExampleConfiguration = dbConfiguration.forPath("example");
// or 
Configuration dbExampleConfiguration = Config.forPath("database.example");
dbExampleConfiguration.get("password");
dbExampleConfiguration.get("username");

Related to #43