allow for expressions in symbol rotation and scale

the single, graduated and categorised symbol renderers have been
refactored to avoid use of raw pointers (auto_ptr are used instead, they
are deprecated in c++11 but the change for unique_ptr is an easy one and
they clearly state the design intend: exclusive ownership)

2 general use function to quote and unquote strings for/from xml have
been added to qgis.h