Handle pom <parent> tag

[arichiardi]

• adds support for the <parent> tag in boot.pom for both writing and reading pom.xml files.
  • also removes some obsolete file and hooks up boot/worker tests in Makefile.

[arichiardi]

I don't remember what I wanted to do in the above comment. It looks like I wanted to split the argument in three (regression aside) but really don't remember what the path was for. In any case the regression should in #578 should probably be addressed first.

[martinklepsch]

Is the basic feature here still wanted? Is that task option type stuff a blocker?

Lets go ahead and merge this if not. I'll write a changelog entry after merging.

[arichiardi]

I think this makes things nicer when writing actual Maven poms for plugins. I have needed this and it does not hurt. I need to actually check the bug though.

[martinklepsch]

Yikes I think I might have somehow wrecked this PR by using Githubs web interface to address the conflicts in Changes.md.

Merged now though

• Change: https://github.com/boot-clj/boot/commit/e7898a99c9ff7c94f35af7b2fa6a2df997b1f3bf
• Tests: https://github.com/boot-clj/boot/commit/94696f66945b640b4d26f45b699342db55786938
• Changelog: https://github.com/boot-clj/boot/commit/49ea7d6207e694f98375f57776846732e3c91908