Migrate to standard updater

[hinerm]

Long-term maintenance of a custom update site protocol is not practical. Ideally we would make use of existing software (e.g. Eclipse update sites).

Our perfect world requirements are:

• Automated uploads via Jenkins
• Fully reconciled with GitHub source
• Downgrade easily (e.g. to Maven releases)
• Dependencies between extensions/"update sites"
• Skew prevenation - e.g. to avoid overriding core ImageJ JARs, and possibly also safeguards to avoid enabling update sites shipping conflicting versions of libraries (like Bio-formats)

Optional but may be nice:

• Automatic life-line version generation
• Generalizeation of "Make Fiji package" to ImageJ2 core

[ctrueden]

Just to clarify: this is not an issue we can tackle this year. But we do want to keep an eye on industry standard update mechanisms for Java software, for the future.