Resolving inconsistent FileSystem.fullPath behavior on different targets.

Current implementations of sys.FileSystem.fullPath differ in behavior on different targets. For example, PHP and Neko follow symlinks, while Python and Java don't. Python doesn't require the path to actually exist on the local filesystem, while other platforms do.

This pull request creates an additional function, sys.FileSystem.absPath. fullPath will find only real paths and eliminate symbolic links; absPath simply joins the path to the current working directory if it isn't already an absolute path, and the path doesn't have to exist.

There's also a new function haxe.io.Path.isAbsolute for checking whether a path is absolute.