[MASSEMBLY-832] Consider dependency optional attribute when gather de…

[zabetak]

https://issues.apache.org/jira/browse/MASSEMBLY-832

Maven allows to declare dependencies as optional and this is important for various reasons (e.g., avoiding licensing problems).

This change provides a convenient way to include/exclude optional dependencies in a dependencySet.

By default optional dependencies are excluded from the dependencySet. An optional dependency shouldn't be packaged, distributed, or transitively inherited unless specified explicitly so it makes sense to exclude them by default.

Following this checklist to help us incorporate your contribution quickly and easily:

• Make sure there is a JIRA issue filed
  for the change (usually before you start working on it). Trivial changes like typos do not
  require a JIRA issue. Your pull request should address just this issue, without
  pulling in other changes.
• Each commit in the pull request should have a meaningful subject line and body.
• Format the pull request title like [MASSEMBLY-XXX] - Fixes bug in ApproximateQuantiles,
  where you replace MASSEMBLY-XXX with the appropriate JIRA issue. Best practice
  is to use the JIRA issue title in the pull request title and in the first line of the
  commit message.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Run mvn clean verify to make sure basic checks pass. A more thorough check will
  be performed on your pull request automatically.
• You have run the integration tests successfully (mvn -Prun-its clean verify).

If your pull request is about ~20 lines of code you don't need to sign an
Individual Contributor License Agreement if you are unsure
please ask on the developers list.

To make clear that you license your contribution under the Apache License Version 2.0, January 2004 you have to acknowledge this by using the following check-box.

• I hereby declare this contribution to be licenced under the Apache License Version 2.0, January 2004
• In any other case, please file an Apache Individual Contributor License Agreement.