Letting uglify-js2 work better with this function

UglifyJS2 with compression enabled and default options will see the line
    "" + this.toString()
and minify it into this line
	"" + this
and that normally should work, but the .valueOf() function then falls into
an infinite loop.  Assigning the same function to both .toString() and
.valueOf() not only eliminates this problem, but it also shrinks the code an
eensy weensy bit.
Merge pull request #60 from fidian/gh-pages

UglifyJS compressing causes Range Error