ENH: Add NAMESPACE file for import specification for R

In the recently tested R version 3.0.2, this NAMESPACE file is now
required.

Change-Id: Ic3824c9c2e03da51d483d0da8ca2a0e33a40e995
Merge topic 'SIMPLEITK-477_AddNAMESPACEFileForR' into next

b2eff9b5 ENH: Add NAMESPACE file for import specification for R