refactor - introduce method to get the debug backtrace
implement php version switch for calling debug_backtrace
mod: use version_compare() instead of PHP_VERSION_ID
     The constant is only available since v5.2.7
add: check for versions above 2.5.2
     Once for completeness, but also to ease future modifications.
Merge pull request #78 from bwoester/fix-debug-backtrace-signature-change

Fix debug backtrace signature change
Merge branch 'master' into dev
Debug backtrace moved to YiiDebug class