Using serial_reduce() when reducing on CPU device

[jszuppe]

There is a bug which causes using reduce_on_gpu whenever reduction function is plus<T>, even if device is not a GPU. Now for CPU devices serial_reduce is always used.

This should fix #476.

Also this change improves performance of reduce algorithm as reduce_on_gpu is not efficient on CPU.

Device: AMD Phenom(tm) II X3 710 Processor (CPU)

=== accumulate with compute === [ MASTER, reduce_on_gpu is used]
size,time (ms)
2,0.179798
4,0.186708
8,0.172284
16,0.187684
32,0.163798
64,0.185535
128,0.173303
256,0.186923
512,0.168256
1024,0.173956
2048,0.200156
4096,0.235499
8192,0.282913
16384,0.664291
32768,0.699428
65536,1.259930
131072,2.511260
262144,4.673290
524288,9.389420
1048576,17.946400
2097152,38.079900
4194304,76.685300
8388608,123.566000
16777216,296.414000
33554432,603.888000

=== accumulate with compute === [ FIX, serial_reduce is used ]
size,time (ms)
2,0.075449
4,0.075187
8,0.076612
16,0.074292
32,0.073260
64,0.072319
128,0.072425
256,0.077508
512,0.068213
1024,0.077216
2048,0.074187
4096,0.077526
8192,0.083586
16384,0.087925
32768,0.103001
65536,0.149885
131072,0.201135
262144,0.307403
524288,0.557656
1048576,1.031420
2097152,2.347310
4194304,4.742430
8388608,9.330300
16777216,18.533900
33554432,36.776200

(This is perf_accumulate but accumulate uses reduce algorithm in this benchmark.)

[jszuppe]

#476 can be closed (I assume you tested it on OSX like you said you'd do).