Fixed a few bugs found when playing with module

  1. pip installation failed (added MANIFEST.in to resolve)
  1. One of the examples (dictionary initialization) failed with NameError. Added method call on `self` rather than global function call to fix.
  1. Added `active=XYZ` kwarg to `__init__` to support one-line definition of `MultiString` objects in case of dictionary initialization.
  1. Raised more specific `MultiString.ContextException` rather than `KeyError` that was thrown when trying to use (dictionary initialized) `MultiString` that had no currently active context.