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
Introduce libevent poll implementation.

Review: https://reviews.apache.org/r/28323
Move poll based socket implementation into poll_socket.cpp.

Review: https://reviews.apache.org/r/28467
Add a factory pattern for constructing a Socket.

Add a factory pattern to allow construction of different
implementations of the socket interface. Remove default constructor
from Socket.

Review: https://reviews.apache.org/r/28670
Virtualize Socket::Impl interface.

Review: https://reviews.apache.org/r/28671
Fixed compilation issue on OS X with libev.
Cleaned up indentation in src/Makefile.

Review: https://reviews.apache.org/r/29372
Refactor fetcher namespace into a class/process.

Layed the groundwork for having a fetcher object with a cache dir per
slave. Factored all fetcher-relevant code into a fetcher class and
process. Added a fetcher parameter to a lot of methods related to
launching tasks, mostly in containerizers. The latter are thus not
holders of fetchers, only slaves are. See MESOS-2172.

Review: https://reviews.apache.org/r/28830
Consolidate all fetcher env vars into one that holds a JSON object.

Changed env var ensemble into one env var containing JSON, parsing
this to protobuf for use. Adapted fetcher tests.

Review: https://reviews.apache.org/r/28975
Added Network Isolator flags to configuration.md.

These flags are visible only when configured with --with-network-isolator
flag.

Review: https://reviews.apache.org/r/29219
Updated help text for max_executors_per_slave flag for master.

The new help text has more context as to why this flag is needed.

Review: https://reviews.apache.org/r/29227