Error handling optimzed

Optimization of the error handling for the case that remote source data
cannot be loaded (fallback to old cached version)
Allow NULL values for updater options

Correction of the option change to allow NULL values to be set for
options (e.g. as default value)
Optimized setting of updater options

The updater options have been set by overwriting the options of the
parent class. This has be change to extend them instead (in the
constructor of the extending class).
Correction of return value and documentation

Correction of the default return value of
\Crossjoin\Browscap\Updater\None::getBrowscapSource() and the method
documentation.