add mode parameter to nssdb::create
add stdlib to .fixtures.yml
replace concept of <basedir>/<dbname> with just <certdir> for flexibility

- replaces dbname, basedir params with certdir in nssdb::create &
  nssdb::add_cert_and_key
- add certdir_mode, manage_certdir params to nssdb::create
- also convert to 2 space indent + linter fixes
add nssdb base class to ensure package deps

This will prevent duplicate package type declarations when using one of
the defined types multiple times in the same manifest.
add nssdb::add_cert define for importing certs into a db

In addition, this changeset is removing this functionality and the
cacert, canickname, and catrust params from the nssdb::create define.
It would be easy to add support back to that define as a wrapper around
the new nssdb::add_cert type or to add a new 'convenience' type to
accomplish the same thing.
add boilerplate .gitignore
change pkcs12 loading to check for existence of the pair in the db
be explicit about ordering between nssdb::create & nssdb::add* types
change pkcs12 file generation to be based on existence of the output file
set umask on generated pkcs12 file so the mode ends up as '0600'
fix nssdb::create manage_certdir => false
add a simple example to README
fix multiple nssdb::create declarations