Use dlopen instead of directly linking to kim-api
library.  This will allow for easier distribution
of LAMMPS with support for KIM Models.

remove attempts to use urllib and use curl for all downloading. restore printing help with no flags.

- build into local directory to replace existing installation is now default
- add wrapper function that calls curl in case python package has not ssl support
- have to specify -n flag to avoid wiping out the existing installation
- can specify -p to point to an existing kim-api installation (implies -n)