Sequential task execution policy

[hkaiser]

This patch

• adds a new execution policy for the parallel algorithms: sequential_task_execution_policy (for sequential asynchronous execution)
• renames task_execution_policy to parallel_task_execution_policy (for parallel asynchronous execution)

Both asynchronous execution policies can be specified by adding the task modifier to the predefined par and seq execution policies. FOr instance:

vector<int> v = { ... };
future<void> f = parallel::for_each(seq(task), begin(v), end(v), [](int v) { ... });

(or par(task), respectively).

All of the documentation and all tests have been updated as well.

[K-ballo]

Looks good to me