Fix text type validation

Merge pull request #14 from czcorpus/text_prop_medium

Add new core text type: "medium"

Fix - forgotten new file

Add support for tagsets in corpora settings

Add missing corpus configuration properties

Add forgotten readme

Add method TokenSlice.String()

Fix - typo in JSON attribute name

Fix - TokenSlice must have a custom JSONUnmarshal

Fix - missing UnmarshalJSON for CloseStruct