Improve groupId, artifactId, name in generated pom.xml

1. Replace hardcoded groupId (`com.wordnik`) with `package` variable.

2. artifactId and name were using non-existent variable `artifactId`.
This results in an empty artifactId, which makes Maven puke.
Replaced them with a sensible hardcoded value, "api-client".
Merge pull request #127 from cb372/fix-pom-fields

Improve groupId, artifactId, name in generated pom.xml