add some details about _content_from_trigger()
make Glow::Role::DirectoryEntry more generic

A directory entry is really just a filename and a digest.
Every other attribute is optional (e.g. 'mode' in Git)

Which means each Glow::Repository needs to implement its own
DirectoryEntry class if it wants to consume the FromDirectoryEntries
content builder.
move Glow::Repository::Git::DirectoryEntry into its own file
use better variable names
documentation for Glow::Repository
document the interactions of Glow::Role::ContentBuilder with Glow::Role::Object
documentation for Glow::Role::Object