Remove:
sequential_vector_execution_policy, seq_vec

Rename:
parallel_vector_execution_policy into vector_execution_policy
std::par_vec to std::vec
Revisited Exceptions section.
Made the following changes:
- an algorithm with an explict policy always throws an exception list
- no system_error is parallel resources are unavailable
- more verbiage around termination
Merge pull request #20 from arturl/master

Fix for issue #15