CCEffectGlass and CCEffectReflection - Fix fresnel calculation

- Change pow(base, exp) to pow(max(base, EPSILON), exp) to avoid raising zero to a
power. This fixes the black dot that was visible at the center of the reflective sphere in
the simple reflection test when run on a device.
- Clamp the fresnel term to the range [0.0,1.0]. This fixes SpriteBuilder Issue #895 in
which nodes within a CCEffectNode will disappear if CCEffectGlass is added to the effect
node.
Merge remote-tracking branch 'upstream/develop' into develop
Merge pull request #1123 from thayerandrews/develop

CCEffectGlass and CCEffectReflection - Fix fresnel calculation