switch from Python sqlite3 module to apsw

[riastradh-probcomp]

http://rogerbinns.github.io/apsw/

apsw supports:

• sqlite3_backup API, so we can save in-memory databases to disk and take reliable snapshots of databases on disk
• virtual tables, so we can obviate the need to create a large intermediate table up-front for SIMULATE
• less obnoxious automagic DB-API transaction nonsense to work around
• anything else in the sqlite3 API we care about

But it also means a nontrivial external dependency.

[axch]

Would ease #238 by providing corresponding modes for the underlying sqlite database, which sqlite3 apparently does not.

[riastradh-probcomp]

Someone^TM should just do this. I started on the 20150210-riastradh-apsw branch https://github.com/probcomp/bayeslite/tree/20150210-riastradh-apsw, but at this point it would probably be better to start afresh using that branch as inspiration than to whittle that branch into shape.

[riastradh-probcomp]

20151119-riastradh-apsw #320

[riastradh-probcomp]

Done in c492cf3. Depend on >=0.1.4a20151203 to get this.