php: attempt at fixing Math.exp specifications

git-svn-id: http://haxe.googlecode.com/svn/trunk@6171 f16182fa-f095-11de-8f43-4547254af6c6
php: minor improvment to Math.exp

git-svn-id: http://haxe.googlecode.com/svn/trunk@6172 f16182fa-f095-11de-8f43-4547254af6c6
mentioned Php Windows bug in Math.NaN (fixed issue #1469)

git-svn-id: http://haxe.googlecode.com/svn/trunk@6173 f16182fa-f095-11de-8f43-4547254af6c6