Added --modules flag for Mesos slave.

Review: https://reviews.apache.org/r/26509
Replaced cerr/exit with EXIT when parsing master flags.

Review: https://reviews.apache.org/r/26629
Fixed minor typos in configure.ac.

Review: https://reviews.apache.org/r/26728
Added DISTRIBUTE reference so that protobuf egg can be built locally.

Review: https://reviews.apache.org/r/26788
Expose poll interval from the reaper.

Review: https://reviews.apache.org/r/26229
Installed mesos pre commit hook and .gitignore on bootstrap.

Review: https://reviews.apache.org/r/26794
Properly qualify process::Failure() in posix.hpp.

Fixed compilation problems when compiling files which include
<slave/containerizer/isolators/posix.hpp> without
"using process::Failure".

Review: https://reviews.apache.org/r/26791
Added "name" to Modules protobuf.

Allow the users to specify a bare library names.

For example, "foo-2.3" will expand to "libfoo-2.3.so" on Linux and
"libfoo-2.3.dylib" on OS X.

Review: https://reviews.apache.org/r/26674
Allowed unloading of a single module.

unloadAll() forces destruction of Owned pointers to the dynamic
library handle. This implicitly causes a dlclose() of all the loaded
module libraries. Unloading a dynamic library can have side effects if
some shared state (e.g., a module) in Mesos points into the library.

Futher more, removed the existing unloadAll() that is no longer
needed.

Review: https://reviews.apache.org/r/26789
Added an advisory message reminding the user to initialize the log.

Review: https://reviews.apache.org/r/26815
Fixed a typo in the last commit.