Add missing log categories

This is to fix superfluous log messages in custom logs. It adds categories to those JLog::add calls where there was no category specified yet.

Short history (tl;dr: skip this paragraph): For my own extension, I wanted to create a separate logfile.
For that reason, I called the addLogger function just as described here: http://docs.joomla.org/Using_JLog. I noticed however that some unrelated messages were cropping up from time to time in the extension's log files ("FinderIndexerAdapter::getTypeId", "Can't identify browser version. [..]"); that was even though I had specified my extension's name as category. Looking through the core code, I realized this was due to those error messages not having assigned a category. That's why I've prepared this pull request for Joomla.

This is my first pull request for Joomla, so please point me in the right direction if there's anything amiss; I've already signed the contributor's agreement (my name is Bernhard Fröhler).

I hope I have chosen the correct branch to base this on - not sure about the branching strategy of Joomla? Are such fixes supposed to go into staging/master first maybe? Problem is some things ("Can't identify browser") were already removed there. And which branches would this need to be merged in addition? I suppose 3.4-dev? Thanks in advance for your patience with a newbie...