Fix enums when using packages

Use of a package name and enums sends was breaking compilation.
protobuffs_compile:atomize/1 is passed three variants of arguments
when using enums and packages:
  "enum_name"
  [ "myenum" ]
  [ "myenum", "my.package_myenum" ]
Add guards to atomize

The first two function clauses were matching one- and two-character
symbol names.  Add guards to ensure the argument is a list of strings,
not just a single string (list).
Merge pull request #49 from d-mart/issue_43

Fix enums when using packages