Ensured "toString" is defined as "Object.prototype.toString"

The code was calling "toString" directly, which resolved to
"window.toString" in browser environments. As "window" is a native
object, there is no guarantee that it inherits from Object. In
particular, there is no guarantee that "toString" is defined and
is a regular JavaScript method.

In particular, Internet Explorer raises a script error when
trying to call "window.toString.call('blah')".

Code updated to actually use "Object.prototype.toString".

The commit also introduces a new "serializeErrorsAsObjects" property
on PatternLayout that determines whether errors get reported as
serialized objects or whether their "toString" method is called. Their
"toString" method gets called by default.
v0.1.9