Auto-namespacing and include_schemalocs

This pull request includes the following changes.

* #163 - Added `auto_namespace` param to `Entity.to_xml()` with default value of `True`. This disables automatic namespace discovery and improves serialization performance.
* #161 - Added `include_schemalocs` param to `Entity.to_xml()` with default value of `True`. Setting this param to `False` remove the `xsi:schemaLocation` attribute from the exported XML.