Added Phalcon\Cache\Backend\Libmemcached

cache backend class by using memcached PECL extension.
Fix #903
The root memory frame survives across requests
Fix 'pointer targets differ in signedness' warning
Fix bug in phalcon_escape_multi() introduced in 1ce094c60dd4a4ef674cbbfe7ae86e22958655e7
Fix #908
Implemented #885
Unit test
Fix #701
Unit test
Commented out listenLowSeverity()
Merge pull request #912 from sjinks/mm

Further Memory Manager optimizations
Merge pull request #917 from sjinks/issue-903

Phalcon\Tag::escapeHtmlAttr() to properly escape HTML attributes
Merge pull request #920 from sjinks/issue-908

Flash\Session::getMessage('key') returns ALL messages
Merge pull request #921 from sjinks/issue-885

[NFR] Make Model::getMessages() filterable
Merge pull request #922 from sjinks/issue-701

Mix integer and string placeholders in Phalcon\Mvc\Model\Query\Builder
Merge pull request #913 from kjdev/1.3.0

Additional: Phalcon\Cache\Backend\Libmemcached
Fix invalid memory reads when GC is on
Fix #802
Fix #910, 914