Simple:
Instead of creating a mirrored animation like "movingright" and "mirrorleft", you only create a "moving" animation.
Than you set the key to adjust the angle:
if left key is pressed, set angle to 180, if right key is pressed, set angle to 0
make sure to check "Auto Mirror" in the player's properties
Here ya go:
dl.dropbox.com/u/23138239/auto_mirror.capx
:)