Add absolute function

[the-real-stiven]

Native implementation of absolute() Graphite function. (http://graphite.readthedocs.io/en/latest/functions.html#graphite.render.functions.absolute)

Speed improvement:

---------- Native targets/absolute.json Latencies ----------
Mean: 6.219059ms
50th percentile: 5.131637ms
95th percentile: 12.810392ms
99th percentile: 19.082978ms
Max: 28.004244ms
Success: 100%
Errors: []

---------- Graphite (Python) targets/absolute.json Latencies ----------
Mean: 21.843168ms
50th percentile: 20.08863ms
95th percentile: 30.953993ms
99th percentile: 46.838977ms
Max: 249.920044ms

Success: 100%
Errors: []

---------- Speed Improvement ----------
Mean: x3.5
50th percentile: x3.9
95th percentile: x2.4
99th percentile: x2.5
Max: x8.9

[the-real-stiven]

@Dieterbe

[Dieterbe]

LGTM
if you use any tools to aid with building these functions, please share them. e.g. can you put them in a gist or in a git repo somewhere?