Include warmup feature in StmBench7:

* A new parameter –warmup specifying the warm up period in seconds.

* class Benchmark:: setupStructures methods is split in two methods: setupStructures and setupThreads.

* class Benchmark::start has two blocks: one for warm up and other to run the benchmark.
Merge pull request #2 from fmcarvalho/develop

Include warmup feature in StmBench7: