Temporarily disabled health check test.

Health check test is flaky and has been temporarily disabled until we
have found the root cause.
Updated Sorter to allow transforming allocated resources.

Review: https://reviews.apache.org/r/29083
Updated Allocator to allow transforming allocated resources.

Review: https://reviews.apache.org/r/29084
Added an overload of Resources::contains for Resource object.

Review: https://reviews.apache.org/r/29134
Added a method in Resources to return all persistent disks.

Review: https://reviews.apache.org/r/29177
Disallowed empty DiskInfo.

Review: https://reviews.apache.org/r/29178
Renamed totalResources in master to offeredResources.

Review: https://reviews.apache.org/r/29179
Adjusted the calculation of unused resources in _launchTasks by
considering persistent disk acquisition.

Review: https://reviews.apache.org/r/28720
Replaced a bad allocator CHECK with a TODO.
Removed an invalid CHECK in the allocator.
Updated the master to transform allocations.

Review: https://reviews.apache.org/r/29234
Added protobuf protocol for communicating persisted resources between
master and slave.

Review: https://reviews.apache.org/r/28723
Fixed a compilation issue in GCC 4.6.

Review: https://reviews.apache.org/r/29180
Refactor clock to simplify libev cut.

Review: https://reviews.apache.org/r/28315
Initialized EventLoop and Clock earlier.

Review: https://reviews.apache.org/r/29225
Add enable-libevent flag to Libprocess.

This flag will be used to distinguish between libev (default) and
libevent.

Review: https://reviews.apache.org/r/28319
Add multiple include guard to libev.

Review: https://reviews.apache.org/r/28320
Introduce libevent header and implementation.

Review: https://reviews.apache.org/r/28321
Add construction and conversion from / to timeval for Duration.

Review: https://reviews.apache.org/r/29226
Introduce libevent clock implementation.

Review: https://reviews.apache.org/r/28322