Fixes #1025, spaces in id-to-taxonomy file break assignment with Mothur.

New pull request based on latest version of QIIME codebase.
There may be a subtle issue with the reverse mapping. Consider the case where there are two original taxon names, `foo<space><space>` and `foo__`. The first would be converted to `foo__` and saved in `self._original_taxa`. The second wouldn't be converted or saved because it doesn't have any spaces.

When performing the reverse mapping here, `foo__` would always be mapped back to `foo<space><space>`, though in some cases it would need to stay `foo__`.