Benchmarks for IDSet.

As of 13/05/2014:

Benchmark incremental build.
Created the permutation array (100000 elements) in 16 ms.
Warm up. Done.
Incrementally added 100000 elements in 1247 ms.

Benchmark build.
Created the permutation array (10000000 elements) in 602 ms.
Warm up. Done.
Build IDSet with 10000000 elements in 1444 ms.

Benchmark remove.
Created the permutation array (10000000 elements) in 576 ms.
Warm up. Done.
Removed 100 elements from IDSet with 10000000 elements in 701 ms.

Benchmark add.
Created the permutation array (10000000 elements) in 566 ms.
Warm up. Done.
Added 100 elements from IDSet with 10000000 elements in 51 ms.

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>