ConfigSchema should provide an interface for constructing from a string

ConfigSchema.__init__() takes a filename argument. This can cause problems if your schema file lives inside an egg. You'd like to use pkg_resource.resource_string() to get the schema as a string, but this can't be passed to ConfigSchema's constructor. Okay, you can use pkg_resource.resource_filename(), but that creates a cache file, which is kind of ugly.

ConfigSchema's constructor should accept a filename or file-like object.