Re-integrates the qDebug messages for cookie jar

Adds qDebug messages to CookieJar constructor.

https://github.com/ariya/phantomjs/issues/11417
Renames m_cookieJarSingleton to m_defaultCookieJar

m_cookeJarSingleton has changed from being the only cookie jar to being the
default cookie jar.
This warrants a name change.

https://github.com/ariya/phantomjs/issues/11417
Adds documentation to cookiejar.js

https://github.com/ariya/phantomjs/issues/11417
Makes CustomPage constructor default parent to NULL.

Changes order of constructor parameters so that the parent is last and can be
defaulted to NULL.

https://github.com/ariya/phantomjs/issues/11417
Makes cookieToMap in CookieJar a public slot.

Previously cookieToMap was a public function.
Since cookiesToMap is a public slot, this change makes the cookie jar
interface more symmetric.

https://github.com/ariya/phantomjs/issues/11417