Merge pull request #221 from LLNL/feature/header-refactor

Refactor headers to allow for more flexible selection of RAJA features. This PR has a number of major changes:

- Headers organized into `pattern` and `policy` directories for patterns and execution policies.
- Header extension changed to .hpp
- Moved all execution policy specific implementations to the `RAJA::impl` namespace.

To include only a subset of features, users can include the policies they want, followed by the patterns they will use, for example:

`#include "RAJA/policy/openmp.hpp"`
`#include "RAJA/pattern/forall.hpp"`

would bring in only support for OpenMP-based `forall` methods.