Moved slave state API functions to the top of the header.

Review: https://reviews.apache.org/r/16237
Paginated all tasks on the web UI home page.

Angular UI already included a Pagination directive that renders a
Bootstrap-styled pagination list. This configures that directive and
adds paginated task tables to the homepage.

* Pagination displays only if there are more items than can fit on a
  single page
* Default itemsPerPage is 50, which will mean 50 Active Tasks and 50
  Completed Tasks on the home page.
Review: http://reviews.apache.org/r/16331
Fixed a bug that can cause the Master to produce a growing number of
leader detection loops.

From: Jiang Yan Xu <yan@jxu.me>
Review: https://reviews.apache.org/r/16333
Added timeval retrieval from sysctl.

Review: https://reviews.apache.org/r/16336
Added os::bootId.

This can be used to identify a reboot, should the boot id change.

For OS X and POSIX systems, the boot time in seconds from sysctl and
utmpx (respectively) are used as a good approximation for a unique
boot id.

Review: https://reviews.apache.org/r/16235
Updated the slave to not recover after a reboot.

Review: https://reviews.apache.org/r/16238
Added equality comparator for Option<T> vs T.

From: Jiang Yan Xu <yan@jxu.me>
Review: https://reviews.apache.org/r/16332
Fixed group to not retry when non-retryable errors are encountered.

From: Jiang Yan Xu <yan@jxu.me>
Review: https://reviews.apache.org/r/16290
Changed master detector to return Future<Option> instead of
Future<Result>.

Now that local session timeouts are transparent to the clients of the
master detector, they are not supposed to retry anymore. Thus passing
"Result<UPID> previous" back into MasterDetector::detect() is no
longer correct.

From: Jiang Yan Xu <yan@jxu.me>
Review: https://reviews.apache.org/r/16291
Updated Group to discard futures instead of failing them upon
destruction.

From: Jiang Yan Xu <yan@jxu.me>
Review: https://reviews.apache.org/r/16345