Add Sequence Bloom Tree-based search into sourmash

[ctb]

Makes use of @luizirber code from https://github.com/luizirber/2016-sbt-minhash to provid indexing and search subcommands.

Interesting question to ask: should we copy the SBT files into this repo? We could either have the SBT stuff be another project or we could develop it as part of sourmash until we have reason to split it off. Alternatively we could develop the SBT code as part of khmer (again, until we have reason to split).

• Is it mergeable?
• make test Did it pass the tests?
• make coverage Is the new code covered?
• Did it change the command-line interface? Only additions are allowed
  without a major version increment. Changing file formats also requires a
  major version number increment.
• Was a spellchecker run on the source code and documentation after
  changes were made?

[ctb]

The lazy loading hack (5a78330) really speeds things up @luizirber!

[codecov-io]

Current coverage is 72.92% (diff: 58.03%)

Merging #45 into master will decrease coverage by 10.34%

@@             master        #45   diff @@
==========================================
  Files             9         11     +2   
  Lines          1106       1869   +763   
  Methods          38         38          
  Messages          0          0          
  Branches         48         48          
==========================================
+ Hits            921       1363   +442   
- Misses          167        488   +321   
  Partials         18         18          

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[luizirber]

Needs some tests for categorize and search, but I'm merging because we are putting too much stuff on this already!