Fix #153 - Log / Error Notification System

Fix #153. Adding log window to "Help" menu to display log messages. Highest priority log message shown in status bar (latest one only), with an icon to indicate the nature of the message.

- [x] Add menu option to "Help" menu (opens empty window currently)
- [x] Set up log window with list and headings
- [x] Create custom log handler class to populate and add to list
- [ ] Add support for filter based on priority
- [x] Add latest high priority message to status bar
- [ ] Add (custom?) icons indicating message nature
- [ ] Double-clicking icon or latest log message shows log
```python
"""Log Dialog for the Freeseer Project.

This should be available as a menu option in FreeseerApp.
"""
```

There's more info here about docstrings for classes
- http://google-styleguide.googlecode.com/svn/trunk/pyguide.html#Comments (see Classes section)
- http://legacy.python.org/dev/peps/pep-0257/#multi-line-docstrings

PEP 257 goes into more detail about docstrings for classes, which we don't currently follow but IMO would improve our codebase if we did.

> The docstring for a class should summarize its behavior and list the public methods and instance variables. If the class is intended to be subclassed, and has an additional interface for subclasses, this interface should be listed separately (in the docstring). The class constructor should be documented in the docstring for its __init__ method. Individual methods should be documented by their own docstring.

> If a class subclasses another class and its behavior is mostly inherited from that class, its docstring should mention this and summarize the differences. Use the verb "override" to indicate that a subclass method replaces a superclass method and does not call the superclass method; use the verb "extend" to indicate that a subclass method calls the superclass method (in addition to its own behavior).