Refactor lammps formula and fix OpenMP implementation

This includes the following changes

- Bump to version 2013.02.14
- Single, unified library build function: more maintainable and easier
  to read
- Support for fortran compiler other than gfortran (use FC)
- Better enforcement of clang
- Fix OpenMP installation
  * depends on 'homebrew/dupes/gcc' because some functions in gcc42 omp
    fail to build
  * Change the open-mpi compiler for this (do not do it for mpich2).
  * This should be done by MPIDependency ( mxcl/homebrew#17370 )
- Roll back incorrect version-number explanation. It turns out that it
   is 'days' after all
- Add voro++ dependency
- 'enable-user-awpmd' no longer requires mpi