a) adds a Python summary provider for NSDate
 b) changes the initialization for ScriptInterpreter so that we are not passing a bulk of Python-specific function pointers around
 c) provides a new ScriptInterpreterObject class that allows for ref-count safe wrapping of scripting objects on the C++ side
 d) contains much needed performance improvements:
    1) the pointer to the Python function generating a scripted summary is now cached instead of looked up every time
    2) redundant memory reads in the Python ObjC runtime wrapper are eliminated
    3) summaries now use the m_summary_str in ValueObject to store their data instead of passing around ( == copying) an std::string object
 e) contains other minor fixes, such as adding descriptive error messages for some cases of summary generation failure

llvm-svn: 151703