Port DissimilarityMatrix/DistanceMatrix delimited text format to skbio.io

**Not ready for merge.** There are still a few things @ebolyen and I are going to discuss today, mainly related to documentation strategy for supported formats in `skbio.io`. Opening this PR now to let the tests run.

Ported delimited text format for `DissimilarityMatrix`/`DistanceMatrix` to `skbio.io.dm`. Added a sniffer for the file format. Refactored parser and test code to be shorter and cleaner.

TODO:

- [ ] properly deprecate `DissimilarityMatrix.from_file`/`to_file` with warnings (they're currently deleted)
- [ ] add changelog mention
- [x] fill out `DissimilarityMatrix.read`/`write` and `skbio.io.dm` docstrings (they're pretty sparse)
- [x] fill in any missing tests for public API (only the underlying readers, writers, and sniffer are tested)
- [x] update test code once #646 is addressed

Fixes #626.
Yep, this is used in two of the tests in this class. See comment for why the 3x3s are duplicated -- basically it allows the test code to loop over all valid files and compare the parsed object to the expected object, where there are two valid 3x3 files that should both parse into the same object.