Add ability to turn off log categories by default (PLAT-564)

Adds an extra boolean parameter to the log category that allows it to be turned off by default, to support the 'recompile with verbose logging' use-case.  This could be combined at a later stage with the ability to modify logging via environment variables or a SHM segment and a signal.
I think that now you can simply do "explicit operator bool() const" i.e. explicit conversion operators (seems supported in 4.6)
https://gcc.gnu.org/gcc-4.6/cxx0x_status.html