make swf filenames configurable

### background

i'm maintaining the [soundmanager-rails](https://github.com/glaszig/soundmanager-rails) gem which packages soundmanager 2 for use with rails' asset pipeline.
one feature of the pipeline is that upon deployment all assets (from the app itself and from any gem used by the app) get copied to an assets folder and digested leading to filenames like `soundmanager2-frkq3rh23l23fg23rh2l3rb2l.swf`.

now, the current implementation uses `sm2.url` and just appends `soundmanager2.swf` which will not work since rails only knows about `soundmanager2-frkq3rh23l23fg23rh2l3rb2l.swf` which in turn leads to flash support not working anymore because the swf url will 404.

this actually appears to be a real issue ;) glaszig/soundmanager-rails#10

### proposal

make swf filenames configurable so that rails users (and probably users of any other system/framework behaving similarly) can do somthing like this:

```ruby
soundManager.setup({
  url:                 '<%= Rails.application.config.assets.prefix + "/swf/" %>',
  flashFilename:       '<%= File.basename asset_path "swf/soundmanager2.swf" %>',
  flash9Filename:      '<%= File.basename asset_path "swf/soundmanager2_flash9.swf" %>',
  flashDebugFilename:  '<%= File.basename asset_path "swf/soundmanager2_debug.swf" %>',
  flash9DebugFilename: '<%= File.basename asset_path "swf/soundmanager2_flash9_debug.swf" %>'
});
```

This resolves into:

```js
soundManager.setup({
  url: "/assets/swf/",
  flashFilename: "soundmanager2-87b55ed5d96bc6413fc998075abf9bdb.swf",
  flash9Filename: "soundmanager2_flash9-ad48f833e03267cc8714ed4b164744f6.swf",
  flashDebugFilename: "soundmanager2_debug-7d3802dd7cd8eedfced11c9b0ad77183.swf",
  flash9DebugFilename: "soundmanager2_flash9_debug-005b763c4fd6c355dea302d7b08d9b2f.swf"
});
```

### solution

see diff. the change is backward-compatible by providing defaults for the config options:

    'flashFilename': 'soundmanager2.swf',
    'flashDebugFilename': 'soundmanager2_debug.swf',
    'flash9Filename': 'soundmanager2_flash9.swf',
    'flash9DebugFilename': 'soundmanager2_flash9_debug.swf',