Added support for some additional C++11 traits.

Added aliases / fallback implementations for the C++11 <type-traits>
std::conditional, std::is_same, std::is_array, std::is_function,
std::remove_reference, std::remove_const, std::remove_volatile,
std::remove_cv, std::remove_extent, std::add_pointer and std::decay.
Note: Due to the lack of variadic-templates in pre C++11 compilers,
the fallback implementation of OFis_function has reduced functionality,
currently only detecting functions with up to three arguments!
Introduced the new explicit boolean type OFExplicitBool.

OFExplicitBool can be used to restrict an overloaded function's
parameter to "real" OFBool values, e.g. pointers and numeric values
can NOT be converted to OFExplicitBool implicitly.
Fixed possibly wrong overload resolution in DSRCodedEntryValue.

The introduction of the "check" parameter, which was added to the
constructor and the "setCode"-method of DSRCodedEntryValue
(see commit a0fe345b533092b4c6b746078217caa13dc85b0c) yielded the
possibility of string literals being automatically converted to
OFBool and therefore being misinterpreted as the value of "check"
instead of "codeMeaning".
Minor fixes (whitespace and line ending errors).
Access the (total) number of frames in an image.

Added support for accessing the value of Number of Frames (0028,0008), i.e.
the number of frames stored in a DICOM file/dataset.