Merge pull request #1 from brucehoff/master

Initial pull request
Merge pull request #2 from brucehoff/master

Rssh
register .C interface

- C code is using .C, rather than .Call
- Mangle R-visible names with prefix C_...
- use registration when loading dynamic library, so symbols
  are looked up only once and can be referenced explicitly
Merge pull request #3 from mtmorgan/master

register .C interface