README.md: add link to ImageJ web site
README.md: use unified ImageJ web site link
README.md: tweak language
README.md: fix typo in groupId
README.md: update pom-imagej version to 2.35
README.md: add SCIFIO & ImgLib2 to deps list

Funny that these two crucial underpinnings of ImageJ were not
included in the list of dependencies. Of course, they should be.
README.md: update link to bug reporting page

The link now goes to the Bugs page of the unified ImageJ web site.