Fix enums when using packages

_Fix for https://github.com/basho/erlang_protobuffs/issues/43_

Use of a package name and enums was breaking compilation.
`protobuffs_compile:atomize/1` is passed three variants of arguments
when using enums and packages:
```erlang  
  "myenum"
  [ "myenum" ]
  [ "myenum", "my.package_myenum" ]
```
Add additional function clauses for `atomize` to handle the variants.