Collections API now available for WinRT.
Also tweaked some things to work on Windows:
 * noexcept -> _NOEXCEPT
 * use native _Tss_t for thread-specific data (grumbles about there being no public API) on WinRT.
 * disable EPUB_COMPILER_SUPPORTS_CXX_DEFAULTED_FUNCTIONS on MSVC, because it doesn't support all cases (doesn't default operator=)
 * Renamed collection.* to epub_collection.*, because Windows sees #include "me" and #include <me> as equivalent, and I use the system <collection.h> (WinRT C++/CX collection types).
 * Properly surrounded libxml2 externs in package.cpp with #if clauses.
 * NativeBridge now returns *this from operator=().