Reenable high accuracy timers

As discussed on the user [mailing list](http://www.open-mpi.org/community/lists/users/2014/11/25824.php) the current implementation of MPI_Wtime defaults to gettimeofday in most cases. This patch proposes a better approach by:
1. reenabling the use of micro-second accuracy OPAL timers
2. Adding support for cycle level timers

@goodell please review. Based on our discussion at SC I made sure that when available we are using the RDTSC-based timers.
The code (in the context) that was in this `#else` block is buggy.  `rdtsc` must be used with a serializing `cpuid` instruction to ensure that you are timing what you think you are timing.  There's an even better pattern that should be used in general, though I'm not sure we can apply it to `MPI_Wtime()` because we don't know if we are being called at the beginning of a timing region or at the end.  For more information see http://www.intel.com/content/www/us/en/intelligent-systems/embedded-systems-training/ia-32-ia-64-benchmark-code-execution-paper.html