GenericValue/GenericDocument: fix alloctaor/Alloactor typos

This patch fixes some misspellings of "allocator" in document.h.
Fixes the Doxygen documentation of GenericDocument as well.
MemoryPoolAllocator: prohibit copying and assignment

The MemoryPoolAllocator implementation cannot and should not be
copied (Rule of Three).  Declare copy constructor and copy-assignment
operator as private.
FileStream: avoid warning about missing initialisation

FileStream::current_ is now explicitly initialized in the constructor.
Avoids reading the garbage value in case of an empty file.