Fixed links in webui for command executor tasks.

Some frameworks don't report an executor ID, but have a task ID. As a
workaround, we can use the task ID when the executor ID is empty.

Review: https://reviews.apache.org/r/11814
Added committers and a java compiler target to mesos.pom.in.

Review: https://reviews.apache.org/r/12087
Killed os::alive because its semantics was somewhat confusing and
caused some flaky tests.

From: Jiang Yan Xu <yan@jxu.me>
Review: https://reviews.apache.org/r/12073
Updated the Reaper to use os::process instead of os::alive.

From: Jiang Yan Xu <yan@jxu.me>
Review: https://reviews.apache.org/r/12074
Fixed bad Java comparison (use equals(), not ==).

Review: https://reviews.apache.org/r/11488