Gave the plugin an "id". Fixed a pom generation problem.

I believe this addresses the missing property file portion of the following issue (which I submitted):
https://github.com/aalmiray/markdown-gradle-plugin/issues/9

This branch also had a change that addressed license generation in the pom, but I backed that change out when I discovered there was an existing pull request that addressed the same issue.

BTW, thanks for writing markdown-gradle-plugin :-)