Add responsive design breakpoint support to Skrollr

Changes:
@@ -64,8 +64,8 @@
Add detection of ~prefix to keyframe attribute regex
Supplants _constant as index 1 of result object

@@ -232,6 +232,13 @@
Read prefix option from config
Convert single prefix into array

@@ -409,17 +416,86 @@
var pushThis is used to determine whether this keyframe is part of current prefix and should be pushed to current keyframe array

var kf.props changed to get property string, converted to object of seperate properties.
Necessary for property inheritance from other prefixes.

Loop through prefixes (if set).
For each prefix, ignore later prefix keyframes.
Check prior prefix keyframes for CSS properties not included in this keyframe and add them to this property stack.

Update constant index (1 -> 2)

@@ -427,7 +503,7 @@
Update offset index (2 -> 3)

@@ -437,10 +513,10 @@
Update anchor1 index (3 -> 4)

Update anchor2 index (4 -> 5)

@@ -513,9 +589,6 @@
Remove _parseProps call - we've already done it now.

@@ -1156,48 +1229,40 @@
Update function _parseProps() to parse the string of a single keyframe property ot object of seperate CSS properties.

@@ -1703,6 +1768,7 @@
add _prefix to private variables.

Signed-off-by: David Benson <dmbenson1978@gmail.com>