Allow multiple schema instances from different config files

[sanjayprabhu]

Say you have defined a staticconf.Schema class. staticconf should have an easy way to construct different instances of the schema from different config files. Currently it reads it from the global namespace which is a singleton.

This is useful for packages which can be instantiated multiple times and need to be configured differently within the same process.

[dnephin]

Reading from different config files isn't really possible, it just reads from a namespace. What we can do is allow config_path and namespace to be parameterized in the constructor, so you could have it load from a different path or namespace. This should effectively be the same thing.