Merge pull request #238 from Hoikas/hdr-work

More Header Cleanups
Add UnicodeBuffer as first class data type and allow plStrings to be constructed with it
Clean up some useless junk from plString
Use plString in plConfigInfo and friends
Remove xtl::istring and xtl::iwstring
Remove an unnecessary extra heap allocation in plString::Format
Replace xtl::delete_ptr functor with lambdas
Remove xtl::copy_if because:
- It is included in C++11
- Nobody was using it anyway >_<