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.
:+1: for snake lower casing. Fixed and also renamed sniffer, readers, and writers to be "private" (i.e., names start with an underscore).