Implement Headlights logging

Requires adobe-photoshop/generator-core#190

Logs should look like this

```
# This event will fire every time Generator is launched. It will give the version number of generator-core
<EventRec doc="0" category="Scripting" subcategory="Generator" name="Generator: Auto startup: generator-core:3.0.0-dev" time="2014-04-15T14:49:06Z" seq="53" />

# One of these events will fire for every plugin that is loaded
<EventRec doc="0" category="Scripting" subcategory="Generator" name="Generator: Plugin loaded: generator-assets:2.0.0-dev" time="2014-04-15T14:49:07Z" seq="63" />

# In this trace "core-version-test" can be thought of as a third-party plugin (It's just a testing plugin I had lying around)
<EventRec doc="0" category="Scripting" subcategory="Generator" name="Generator: Plugin loaded: core-version-test:0.0.1" time="2014-04-15T14:49:07Z" seq="82" />

# This fires whenever asset generation gets enabled for a document.
# That can happen either by a menu selection, or by simply opening a document for which it is already enabled.
# Since this event isn't preceded by a "menu selected" event, that means it was automatic. 
<EventRec doc="0" category="Scripting" subcategory="Generator" name="Generator: Enable Image Assets" time="2014-04-15T14:49:21Z" seq="89" />

# This event fires when we start creating an asset. It will only fire once per PS session
<EventRec doc="0" category="Scripting" subcategory="Generator" name="Generator: Generated one or more image assets" time="2014-04-15T14:49:22Z" seq="94" />

# This event indicates that a menu named "generator-assets" was selected by the user. This is the menu for the asset generation plugin.
<EventRec doc="0" category="Scripting" subcategory="Generator" name="Generator: Menu selected: generator-assets" time="2014-04-15T14:49:47Z" seq="98" />

# This event fires when asset generation gets turned off for a document.
# This can happen either by a menu selection (as is the case here, since it was preceded by the "Menu selected" event,
# or, simply by closing the document (since we obviously stop making assets for documents that are closed)
<EventRec doc="0" category="Scripting" subcategory="Generator" name="Generator: Disable Image Assets" time="2014-04-15T14:49:47Z" seq="99" />

# This is an event that fired because a third-party menu was chosen. The name of that menu was "core-version-test"
<EventRec doc="0" category="Scripting" subcategory="Generator" name="Generator: Menu selected: core-version-test" time="2014-04-15T14:49:50Z" seq="106" />

# Another file with asset generation enabled was opened and later closed. Assets were generated, but the "Generated one or more assets" event
# did *not* fire, since it had already fired once this session.
<EventRec doc="0" category="Scripting" subcategory="Generator" name="Generator: Enable Image Assets" time="2014-04-15T14:50:18Z" seq="122" />
<EventRec doc="0" category="Scripting" subcategory="Generator" name="Generator: Disable Image Assets" time="2014-04-15T14:50:25Z" seq="132" />
```