Add RAII Exiv2::Library to init/free resources

[piponazo]

As it was discussed in #977 , the idea would be to add in master a RAII class named Exiv2::Library (or better name) which would take care of allocating/freeing library resources in a RAII fashion. At least we should have there the calls to the XmpParser::initialize() and XmpParser::terminate in case the XMP support is enabled.

[clanmills]

We should investigate this for v1.00. As we have a new API for v1.00, we can "clean up" some of those legacy issues.

I know Andreas was strongly against having a library initialisation function. His argument is that all that an application needs is to include <exiv2/exiv2.hpp> and it can call Exiv2 library functions. And he's right. That will work. However, it's better architecture to have process singleton.

If a developer is using Exiv2 in a large/complex application, he may be unable to modify 'main' to host the singleton. That's regrettable. It's not catastrophic.

The use of the singleton is desirable. It should not be compulsory.