Simplified creation of temporary DICOMDIR file.

Replaced creation of a temporary DICOMDIR file by a simpler approach: Just
append ".tmp" to the DICOMDIR filename (in the specified output directory).
This also makes sure that on Windows systems directory names with special
characters (in UTF-16 encoding) are supported.
Added support for some C++11 types, literals and traits.

Updated implementation of fixed-width integer types to better conform the
C++11 standard. This includes OFintptr_t and OFuintptr_t as integer types
capable of holding a pointer-value.
Implemented aliases for C++11's nullptr / nullptr_t (OFnullptr / OFnullptr_t)
an the respective fallback implementations for older compilers.
Introduced new header file "oftraits.h" as pendant to C++11's <type-traits>,
currently implementing integral constants, enable_if and is_signed /
is_unsigned. See http://en.cppreference.com/w/cpp/header/type_traits for
details.
Optionally disable isFilenameValid() check.

Added optional parameter to checkDicomFile() and loadAndCheckDicomFile()
that allows one to disable the check for a valid DICOM filename (used as the
value for ReferencedFileID within the DICOMDIR). This is because the file
might be renamed after one of these methods is called.