Changing base model `Model` to `SQLAlchemyBaseModel`.

Just want to be clearer.
Configuring the SQLA scoped session the old-fashioned way.

It doesn't look like `lib/sqla.py` is used anywhere anymore. Leaving it where
it is just in case.
Fixing User.password method params
Adding dev db file to gitignore.

The development database file shouldn't be part of the source code.
Commenting some legacy configuration.

I'm not entirely sure what these libraries do or which of these views are still
useful, but I'm not ready to throw them away entirely.
Finding parents and children of email posts at content __init__
Fixing post.parent relationship.

There was an error when trying to assign a post's parent post because a Post is
not list-like. I had the backref in the wrong place.
Making email.import_content(only_new=True) the default.
Split synthesis.models.TableOfContents into Discussion and TableOfContents.

- All discussions will create a ToC at init
- discussions have sources, tables of contents have <blank>
- the model with restricted access is the Discussion instead of the ToC
Adding 'ancestry' field and methods to Post.
Using `post.set_parent(foo)` instead of `post.parent = foo`

Maybe I should make `set_parent` into the setter for `post.parent`, but I don't
know how much I like blurring the lines between properties and methods.
Making the creation of new emails easier to look at and understand.
Replacing email.send_date with content.creation_date
Adding field post.is_seen.
Removing redundant import

TableOfContents isn't used in this module.
Fetching by UID in Mailbox.import_email()

Fixes an error when using `mailbox.fetch` on GMail's inbox.
Fixing bug where Mailbox imports emails with empty bodies.
changin post.is_seen to post.is_read
Sorting Post.children by their contents' creation date.
Sorting Post.get_descendants() by their contents' creation_datewq