adds in functionality for direct download and dct references parsing
change strings in constants to symbols
Dynamically resolve reference types
Only intercept for methods defined as constants
Merge pull request #149 from geoblacklight/gravesm-download-types

Dynamic reference type resolution
remove direct_download stub
adds in iiif to constants
Merge pull request #143 from geoblacklight/download-types

adds in functionality for direct download and dct references parsing