Minimum MPI Version Supported

cc: @andreasnoack , @ViralBShah , @eschnett , @amitmurthy

A pull request from @steven-varga with an MPI-3 function brought up this issue. What should be the minimum supported MPI version for this wrapper?

Right now we are using some things from MPI-2 for the types so that is probably as low as we should go but should we require MPI-3? From [this document](http://meetings.mpi-forum.org/mpi3-impl-status-Nov14.pdf) it looks like most major MPI implementations have MPI-3 support or are planning it. The major exception seems to be Microsoft MPI.

We can at compile time determine the MPI version with the constants `MPI_VERSION` and `MPI_SUBVERSION` --- see [MPI-1.3 Standard](http://www.mpi-forum.org/docs/mpi-1.3/mpi-report-1.3-2008-05-30.pdf) section 7.1.1 for more info --- so we could only bring in MPI-3 functions if the library supports it but it makes the wrapper a little more complicated.

Does anyone have any thoughts or concerns on this front?