simplify github event handling.

[nathan-at-least]

Github event processing can be simplified and made more consistent to improve maintainability.

What to Change

First, every event has three common sections according to the docs for payloads: sender, repository, and organization, so these details could be formatted generally for all messages.

Next, it may be possible to replace specific event handling functions with standard string.format templates, with two caveates: incorporating multiple items from JSON arrays and special field formatting, such as for URL-encoding ^ characters (see #11).

The Current Design

Currently there is a FunctionTable mapping github event names to arbitrary functions which return formatted messages. Every such function uses python %-interpolation and explicitly selects pieces of the JSON message (using _AttrLookupWrapper). Since the string.format expansions already support attribute lookup, we can replace these functions with simple string templates.

Features Which string.format Lacks

• Transformations on individual fields. Currently the formatters do things like len(einfo.commits) (see github.py line 112) or truncating long strings as in line 138.
• Formatting each item. In the future we might want to display details about each item in a JSON array, such as each commit's message on a push event or each wiki page modified.

[nathan-at-least]

Adding underspecified tag: this sounds like multiple features/tickets:

• Process sender, repository, and organization in common code for all Github events.
• Announce those three fields identically in all announcements.
• Use str.format. This is already done, see #16.
• Add magic to be able to specify complete event handling as str.format templates. This is partially done?

In any event, some of those features will be obviated, or their implementations drastically altered by #21 which is ahead in the roadmap (0.2.2 versus undecided).