Introduce thrust::system::<name>::par
Introduce thrust::detail::execute_with_allocator
Port custom_temporary_allocation example to use thrust::cuda::par
Split thrust/execution_policy.h into thrust/execution_policy.h and thrust/detail/execution_policy.h
Add thrust::host & thrust::device.
Move the definitions of device_execution_policy & host_execution_policy to thrust/execution_policy.h
Eliminate use of thrust::retag in minimal_custom_backend example.

Fixes #337
Replace <thrust/memory.h> with <thrust/execution_policy.h> in special_types.h.
Merge pull request #343 from jaredhoberock/seq_and_par

Seq and par