moving pristine code to functions/gemset
run pristine only on gems with extensions when in reinstall mode
cleanup, move gemset_reset_env a bit above to group pristine and lsit functions
add --skip-pristine param and rvm_skip_pristine_flag
Merge pull request #2486 from wayneeseguin/features/smaller_pristine_gh_issue_2486

add reinstall option to avoid running gem pristine on all gemsets