Made Image parameter of FlxTrail optional

Image is no longer a required parameter of FlxTrail, if none is
provided, it uses the same graphic as the sprite it follows
Add property "solid" to FlxTrail

Make all trailsprites non-solid by default in order to fix a bug with
collisions, add setSolid() and getSolid()-functions to be able to change
_solid
Merge pull request #295 from Gama11/flixelNME

Made Image parameter of FlxTrail optional / Fix of collision bug