[Curves] new interface for Curves

+ cleaner, nicer interface for preparing numerical curves for Unity
+ simple interface that will work on one variable only
+ no additional dependencies
+ example CurveInstant for things like z-sorting and object-enabled
+ this will be used later on to add ease-in, ease-out curves
[Curves] linear interpolation curve

+ CurveLinear as linear interpolation for given points, with tests

+ fixed amount of return values from internal function
__get_kv_point_at, now always returns 3, no matter if the key belongs
to the dictionary
+ equality, non-equality and str operators added to KVPoint
+ renamed test for CurveInstant
[Curves] linear angle interpolation

+ added part that allows simple angle interpolation
+ uses amazing trick with atan2(sin, cos) to be able to work on any
angles, despite them being bigger than 2pi etc etc
+ set of simple tests to check whether everything is fine
+ note that add_point was overloaded, it’s wrapping given angle at
start, so there is no later problem with interpolation
[Curves] method for listing all available keys

+ some further hiding implementations
+ able to list all available keys from given instance
[Curves] dummy curve

+ a curve that always returns the same value, over and over
+ slopes are always 0.0
+ could be useful for things like z-value for scale or for x and y
values for rotation quaternion
[Curves] filling init

+ added auto import for curves when importing module
[Spriter] support for file-less sprites

+ some objects in scml can have NO files attached
+ since it’s hard to skip them, i’m adding nodes with no sprites
attached
+ sprites are also skipped from animation script
+ should have no impact on everything old
[File] default pivot y

+ pivot_y have to be 1.0 by default, despite what can be seen in spriter
[Saving] unicode utf-8 files

+ animation clip and prefab saver are now using python codecs to save
everything as unicode
+ the rest of the code, suprisingly, works fine with unicode