Improved BezierPlugin, fixed issue fontWeight conflict in CSSPlugin, and an issue that caused strings to be interpreted as relative in raw tweens

- Now the BezierPlugin's "thru" feature will automatically recognize
when the beginning and ending values match and smooth the transition
for seamless looping.
- Fixed issue that could cause an error when "fontWeight" was tweened
with CSSPlugin
- Fixed issue that caused strings to always be interpreted as relative
(like they were prefixed with "+=" or "-=") in raw tweens (this did NOT
affect CSSPlugin tweens)