Exiv2::Value subtypes support

[gogiel]

It should be possible to read and write Value using subtypes, especially XmpValue and its subtypes. Or at least provide access for value_ fields (http://www.exiv2.org/doc/classExiv2_1_1XmpArrayValue.html#620cf5acd40bea72c7b3cdb3894fea6a, http://www.exiv2.org/doc/classExiv2_1_1LangAltValue.html#e466310e854d707c1e5f5a44730a27c9 and others).

If you don't want to break current interface my idea is to add singleton method to the string object that represents Exiv2::Value that will return value_ field if available.

[gogiel]

Exemplary implementation: https://github.com/gogiel/exiv2/commit/35cadeb325bd9e4f7ff5a54ca4a9a2cd4c314440 By the way spec image with description "Pregnant woman shown in nudity." is at least surprising ;)