[kcalcore] "meego port" version of kDebug require to be explicitly turned on

Especially Mkcal is quite chatty on debug output. Avoid flooding the
console by requiring KCALDEBUG environment variable to be set
non-empty.
Merge pull request #4 from pvuorela/master

[kcalcore] "meego port" version of kDebug require to be explicitly turned on