Rename thrust::dispatchable to thrust::execution_policy
Rename thrust::detail::dispatchable_base to
thrust::detail::execution_policy_base
Rename most "System" template parameters to "DerivedPolicy"
Promote thrust/detail/execution_policy.h to thrust/execution_policy.h
Eliminate system/*/detail/adl_targets.h
Introduce system/*/execution_policy.h
system/*/memory.h no longer needs to #include ADL targets at the bottom of the header.
Eliminate typo
Rename system/*/detail/tag.h to system/*/detail/execution_policy.h
Actually rename system/*/detail/tag.h to system/*/detail/execution_policy.h

Fixes #334
include cpp's execution_policy.h instead of memory.h inside TBB's reduce_by_key implementation.
Merge pull request #335 from jaredhoberock/execution_policy

Execution policy