Add accumulate argument to PathAnimationEffect constructor

This allows the accumulation attribute of a PathAnimationEffect to be set,
which previously was not possible.

Note that the patch [1] to fix this in the spec narrowly missed FPWD.

[1] https://dvcs.w3.org/hg/FXTF/rev/f55c175711df
Merge pull request #296 from steveblock/passAccumulateToPathAnimationEffect

Add accumulate argument to PathAnimationEffect constructor