Read text files along with their annotations
Some minor helpers working with paired text
Don't use text as a name; clashes with field
Reorganise corpus reading a bit:

- Discard the notion of a file pattern
  It's not really flexible (what if you want to match on disjunction
  of authors, say?) and not really all that helpful to introduce a
  new concept if Python hackers can just use dict comprehensions or
  lambdas

- Encapsulate corpus functions in a Reader class
  Not entirely sure if this is a good idea (no real OOP skill here).

- Use dataset to hide generic details (eg. educe.stac provides
  stuff from educe.annotation and educe.corpus)