Implementing literature module

Just the first few steps. Not finished yet.
Deletes citation file

The citation class is now in literature file (issue #27)
Improves the literature module

Now the classes are separated into slot classes and the actual
class that inherits from superclass to get ordering that is
important for the scheme. (issue #27)

NOTE: The other files are only changed as trailing spaces are
removed.  My vim is configured like this.
Merge branch 'master' of git://github.com/ropensci/reml
Merge branch 'master' into litrature

Conflicts:
	R/additionalMetadata.R

Resolved:
	Only due to white space removing on my side
Further improves the literature module

o Now the classes have slots like this for right sorting
	- id_scope
	- resourceGroup
	- slots
	- referenceGoup
o More atomic classes

This is related to issue #27
Improves the literature module

o Adds coercion methods XML <-> s4
o Adds BibTeX citation classes and fields as mapping draft in comments

Belongs to issue #27
Merge branch 'master' of git://github.com/ropensci/reml
Merge branch 'master' into litrature
Adds missing bracket in call to constructor
Adds validation for list of attributes
Improves the literature module

o adds first check for required fields in citation types
o adds prototype on citation for eml
Removes trailing whitespace
Removes the validation from article

o Also improves the bibentry class but still in
  comments

Related to issue #27
Merge pull request #70 from cpfaff/litrature

Merging literature module from @cpfaff (to be followed by manual check and merge against devel).