Add a <dependencyManagement> section to pom.xml

This is only one part of a bigger plan to clean up the loose ends of the Fiji projects (well, at least the responsive ones):

* move `pom-fiji` into its own repository
* get rid of all `groupId` properties
* release a `pom-fiji-2.0-beta-1`
* switch all components to release versions
* release a `pom-fiji-2.0`

Steps unrelated to `pom.xml`:

* move `meta-fiji-external-plugins` into its own repository
* make a Jenkins job that automatically uploads from the `meta-fiji-external-plugins`
* split out `fiji-scripts`
* move the scripts not specific to Fiji to the `scijava-scripts` repository (as the vast majority of the code in `bin/` is written by yours truly, with most of the rest being written by @ctrueden and other developers who are more than willing to relicense to BSD for the greater benefit, there is no problem)
* abandon `Build.sh`
* clean out the working directory of `fiji.git` and leave only a `README.md` there
* describe -- briefly! -- how to develop projects with Eclipse in the Fiji/ImageJ Wiki
* link to that Wiki page from the `README.md` file

In essence, this PR is the first step toward relieving the Fiji maintainer from most of the burden of the current process.