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.
Watch out for integer overflow here.  I think you want to convert the fraction to a `double` and then multiply the absolute time by that.  See this detailed Stack Overflow answer: http://stackoverflow.com/a/23378064/158513