Allowed specification of mininum slot count.

To ensure Hadoop jobs begin promptly, we can specify a minimum number of
'hot slots' to be available for use.  This addresses the TaskTracker
spin up delay that exists with Hadoop on Mesos.  This can be a nuisance
with lower latency applications, such as ad-hoc Hive queries.

Review: https://reviews.apache.org/r/12921
Get flags for mesos-*.sh from build directory.
Added link to executor sandbox from framework template

The framework page is rendered with info from the master, but the
sandbox directory is known only by the slave. To link to the directory
from a page rendered by the master, create a page in between that acts
as a redirector by first asking for the directory from the slave.

If the sandbox is important for linking, the master could return
`executor.directory` in the `state.json` endpoint and negate the need
for this redirection stuff.

Review: http://reviews.apache.org/r/13006
Refactored Hadoop on Mesos from contrib to JAR.

Review: https://reviews.apache.org/r/13225
Get Hadoop code to compile!

Review: https://reviews.apache.org/r/13226
Updated README.md.

Review: https://reviews.apache.org/r/13231
Removed Hadoop from repository.

Review: https://reviews.apache.org/r/13245
Fixed slave to properly handle terminated tasks that have pending
updates.

Review: https://reviews.apache.org/r/13187
Improved StatusUpdateManager's API by flattening the return types.

Review: https://reviews.apache.org/r/13196
Fixed slave to remove a queued task when the task is terminated.

Review: https://reviews.apache.org/r/13207
Refactored slave recovery so that slave recovers its state before
the isolator and status update manager.

Review: https://reviews.apache.org/r/13216
Fixed cgroups isolator to not try to destroy already destroyed
cgroup during recovery.

Review: https://reviews.apache.org/r/13224
Added os::sendfile that masks SIGPIPE.

Review: https://reviews.apache.org/r/12619
Updated process.cpp to use os::sendfile.

Review: https://reviews.apache.org/r/12620
Added a Process.provide test.

Review: https://reviews.apache.org/r/12621
Implemented Rate limiter abstraction.

Review: https://reviews.apache.org/r/12018
Added os::process and os::children that uses a Process list.

Review: https://reviews.apache.org/r/12428
Fixed os::sysctl::string to return results with null bytes.

Review: https://reviews.apache.org/r/12429
Added "argv" lookup for Process::command on OS X.

Review: https://reviews.apache.org/r/12430
Added os::pstree, os::pstrees, and os::ProcessTree.

Review: https://reviews.apache.org/r/12431