cpp: Add doxygen API reference
travis: Build cpp doxygen documentation
cpp: Create shell fragment with configured version metadata

This can be sourced by any script which needs to know which
bioformats version it's working with.
cpp: Doxygen generates correct include paths
cpp: Update graphviz version requirement in README
cpp: doc: Update Doxygen configuration
cpp: Document all namespaces, functions and typedefs
cpp: Correct Color.h documentation comments
cpp: Positive constraints are strictly positive
cpp: Move namespace docstrings from Enumeration to EnumerationException

Also fix spelling at the same time.
cpp: ome::xerces::string: Correct spelling
Merge pull request #723 from rleigh-dundee/cpp-doxygen

cpp: Add doxygen API reference