Merge pull request #77 from cpfaff/litrature

Improves the literature module
Adds class union

o This allows for a editor to be listed in creators
  (edited book)

setClassUnion("ListOfcreatorOreditor",
		c("ListOfcreator", "ListOfeditor"))

Related to issue #27
More literature coercions

o Adds more coercions to bib entries
o Adds coercions in party module
Updates to the eml() constructor method

Working on depricating the eml() constructor.
- data.set vs data.frame, and use of `meta` argument, and the prompt for
  the wizard are all handled by eml_dataTable now, so ignore handling
  these.
- Revoked version argument -- cannot handle versioning merely by
  changing namespaces, even though 2.1.1 and 2.1.0 are largely
  compatible.
- If creator is NULL instead of character(0) on account of a call
  to get default from eml_config, reset it to the valid character(0)
  type to avoid throwing an error. It is okay to construct eml
  peicewise and have missing required elements at the start.
tweaking eml constructor more.  updated tests; all tests passing again
advanced writing tutorial (first draft)
fix highlighting for github