Add exclusive_scan for Boost.Compute
Benchmark Boost.Compute and CLOGS versions of sort/scan

@kylelutz, @bmerry, these are results I get on a Tesla K40c:

Sort (ulong)
    VexCL:         3.66220e+08 keys/sec
    Boost.Compute: 1.03752e+08 keys/sec
    CLOGS:         2.26646e+08 keys/sec

Scan (ulong)
    VexCL:         1.96502e+09 keys/sec
    Boost.Compute: 2.13827e+09 keys/sec
    CLOGS:         5.01587e+09 keys/sec

Sort (uint)
    VexCL:         8.16260e+08 keys/sec
    Boost.Compute: 2.00985e+08 keys/sec
    CLOGS:         4.34790e+08 keys/sec

Scan (uint)
    VexCL:         3.93135e+09 keys/sec
    Boost.Compute: 4.48308e+09 keys/sec
    CLOGS:         7.60791e+09 keys/sec