Use artifact-id and group id from parent pom if missing

Fixes #132; requires re-indexing

Conflicts:
	src/clojars/maven.clj
Also inherit version from parent section of POM

As per https://maven.apache.org/guides/introduction/introduction-to-the-pom.html#The_Solution

Note we still aren't fully correct as we're not inheriting
contributors and dependencies. That'd require fetching the parent
POM file.

Fixes #132; requires re-indexing