Fixes #40 by adding a config property "baked.timestamp.as".

When this property is defined in any config, the value is used as key for an element that is added to each file content map.
This way, the date at which this file was parsed can be used to render template.

In other words, suppose I define my config such as

     baked.timestamp.as=timestamp

I can then use in any freemarker template the various [date built-ins](http://freemarker.org/docs/ref_builtins_date.html), such as

     ${timestamp?string.long}

Obviously, if `timestamp` is already used in a file, it won't be overwritten by this config, and a log statement will be emitted.