Clean up the execution mechanism of examples

Motivation:

- There's no way to pass an argument to an example.
- Assigning a Maven profile for each example is an overkill.
  It makes the pom.xml crowded.

Modifications:

- Remove example profiles from example/pom.xml
- Keep the list of examples in run-example.sh
- run-example.sh passes all options to exec-maven-plugin.
  For example, we can now do this:

    ./run-example.sh -Dssl -Dport=443 http-server

Result:

- It's much easier to add a new example and provide an easy way to
  launch it.
- We can still pass an arbitrary argument to the example being launched.
  (I'll update all examples to make them get their options from system
  properties rather than from args[].