Initialize migration management

[rod-glover]

Resolves #31

Installs and configures Alembic.
Establishes initial migration from empty schema to populated schema (tables only).
Respects specified schema name.
Establishes migration test framework and first migration tests.

Upcoming migrations will install functions, views, matviews. One thing at a time.

Guide for the perplexed:

• TL;DR:
  • Review README, alembic/versions/522eed334c85_create_initial_database.py, tests/alembic_migrations/test_migrations.py.
  • The rest is mostly boilerplate.
• There are several hundred lines of new documentation on Alembic, its configuration in this project, and how to use it in this project.
• The alembic configuration proper (everything in subdirectory alembic except for alembic/development/, plus alembic.ini) is quite straightforward and pretty much copied from modelmeta.
• I included a local development database utility (housed in subdirectory alembic/development/) that proved very helpful for coding and testing. It's not central to this PR, but it will continue to be useful.
• The initial migration (alembic/versions) is straightforward.
• Writing tests for the migrations that respect schema name specification required modifying some of the utilities provided by alembic-verify and sqlalchemydiff. No big deal, but more code, at present housed in tests/alembic_migrations/alembicverify_util and tests/alembic_migrations/sqlalchemydiff_util.

[rod-glover]

@jameshiebert , if you have time for a quick look over this code, that would be great.

[rod-glover]

Merging following brief discussion with @jameshiebert