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