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

New pull request based on latest version of QIIME codebase.
Yes, I see, I'll add a test to cover an example like this.  In other words, taxon "a_b_c" and taxon "a b c" should be considered distinct.

I feel like we need a spot to document our policy with ID-to-taxonomy files, don't know quite where to place it in the documentation.

Just to note, current practice for ID-to-taxonomy files and assignment results is to strip leading and trailing whitespace from taxa (see [here](https://github.com/biocore/qiime/blob/master/qiime/assign_taxonomy.py#L109) for id-to-taxonomy and [here](https://github.com/biocore/qiime/blob/master/qiime/parse.py#L421) for assignment output).