Attribute descriptor
Attribute now can has decoders

Attribute becomes a subtype of CodecDescriptor
Use descriptor instead of XML id for internal id
Content now can have decoders
Some docstring for CodecDescriptor
Add MIT license file
Classifiers for package index
read() replaces document's constructor parameter
Make descriptor indexers independent from handler
Missing assertions about ElementList
Prettier repr() for ElementList
DescriptorConflictError for duplicate descriptors
Index all used XML namespaces
Intermediate commit: schema writer
Writing attributes in canonical order for testing
Missing docstring for write() function
Missing docstring for canonical_order parameter
Test write() using DOM
Fix bug that crashes when there's non-ASCII chars
Test non-ASCII chars for attrs & contents as well