Fixed a crash in the ProcessIsolator.

Review: https://reviews.apache.org/r/12179
Fixed a bug in master to properly remove a non checkpointing
framework from a checkpointing slave.

Review: https://reviews.apache.org/r/12182
Changed slave command line flag 'safe' to 'strict'.

Review: https://reviews.apache.org/r/12084
Removed bin/mesos-build-env.sh.in in favor of just putting flags in
the bin/mesos-master.sh.in, bin/mesos-slave.sh.in, and
bin/mesos-local.sh.in scripts since some binaries require different
flags and they will fail on startup if they try and load an unknown
flag.

Review: https://reviews.apache.org/r/12118
Moved master/slave HTTP route handlers into inner class.

Review: https://reviews.apache.org/r/12119
Added help pages to libprocess.

Review: https://reviews.apache.org/r/11710
Updated routes in Mesos to provide help.

Review: https://reviews.apache.org/r/12120
Refactored HierarchicalAllocatorProcess to use roles instead of users.

Review: https://reviews.apache.org/r/11205/
Fixed a compilation issue introduced by
https://reviews.apache.org/r/12119/.
Added a concept of 'valid' roles.

Review: https://reviews.apache.org/r/11206/
Added a json http endpoint for getting information about roles.

Review: https://reviews.apache.org/r/11210/
Added weights to DRFSorter.

Review: https://reviews.apache.org/r/11212/
Added tests for weighted DRF.

Review: https://reviews.apache.org/r/11214/
Updated docs to reflect changes to the allocator.

Review: https://reviews.apache.org/r/11529/
Changed the default AWAIT_* timeouts from 5 seconds to 10 seconds.

Review: https://reviews.apache.org/r/12234
Refactored os::Process to use Options for memory and CPU information.

Review: https://reviews.apache.org/r/12215
Fixed a crash due to invalid utime / stime values coming from /proc/pid/stat.

Review: https://reviews.apache.org/r/12221