Asset compiler refactor

Add events for asset concatenation and minification. These events
can be called individually to trigger the minification and/or
concatenation of assets during application runtime.

To trigger minification:

app.emit('asset_compiler_minify', newMinifyConfig);

To trigger concatenation:

app.emit('asset_compiler_concat', newConcatConfig');

To trigger both minification and concatenation:

app.emit('asset_compiler_minify_concat');

The config object passed to each event is optional.