Get rid of .gitignore files in assertions/ and static/.

Unfortunately, these .gitignore files prevent the files in them from being
present after installing badgehost via npm, which means that one has to
manually copy the files in them from somewhere else to get the standard
demo assertions/images, which is very useful for testing.
Set defaults for the static and assertion dirs.

This will allow clients to simply build the app with no options in their
test suites and get a basic badge server without having to manually configure
anything. They can still provide their own custom values for the directories,
of course, but the defaults allow them to get started faster.
Added app.baseURL and app.url() method.

This intercepts the calling of app.listen() to remember what port the
app is listening on, and setting app.baseURL to include this value.
app.url() is like app.path() but provides absolute URLs to resources,
which is means that programs importing badgehost can very quickly set up
and use a badgehost instance, e.g.:

```
var app = require('./').app.build();

app.listen(function() {
  var url = app.url('demo.json', {set: {stuff: 1}});
  console.log("get your badge at", url);
});
```
Document app.url() in README.
Merge pull request #2 from toolness/master

Make it easier for clients to use badgehost programmatically