Changes to docstrings, package layout, imports.
+ Moved all sdv exceptions to sdv.errors module.
+ Reworked imports to require more namespaced attribute/class
  access. (Less `from foo import bar` and more
  `import foo; foo.bar`).
+ Added sdv.validate_xml(), sdv.validate_profile(), and
  sdv.validate_best_practices() methods for easier validation
  calls. UNTESTED.
+ Added docstrings to more public methods.