Add serializer option to Memcached Cache engine

Add option to select a serializer engine among php, json or igbinary
Throw exception when selected serializer is not installed
Remove unnecessary sprintf
Fix test
Rename `serializer` to `serialize`
Add doc about the new serialize setting
Updated doc block readability in bootstrap.php
Merge pull request #1668 from bcrowe/hotfix-bootstrapdocblock

Updated doc block readability in bootstrap.php
Use instanceof instead of is_a() in Controller.php
Merge pull request #1669 from bcrowe/hotfix-instancecheck

Use instanceof instead of is_a() in Controller.php
is_a() to instance of - completes PR 1669
Merge pull request #1672 from dereuromark/master-instance

is_a() to instance of - completes PR 1669
Replaced all is_a() calls with instanceof operator
Replaced true asserations with instanceOf asserations in test cases
Missed a couple instanceOf asserations
Merge pull request #1673 from bcrowe/hotfix-instanceof

Replaced all is_a() calls with instanceof operator
Resolves ticket #4100
Wrong signature.
Merge pull request #1674 from dereuromark/master-disabled-radio

Resolves ticket #4100
Merge pull request #1675 from bar/patch-1

Wrong signature.