Change behaviour of file field.

* Ask one file at a time when editing the file field. Use TAB
  completion.
* Remove ebib-file-regexp
* New user option ebib-filename-separator
* split contents of file field on ebib-file-separator
Add explicit option "Open in Emacs" to File Associations.
Add history variables.
Small corrections to the manual.
Change file viewing.

* If there are multiple files in the file field, ask the user which one
  to open, unless s/he provided a numeric prefix argument.
Change URL browsing behaviour

* If there is more than one URL in the `url' field, ask the user which
  one to open.
Merge branch 'master' of github.com:joostkremers/ebib

Conflicts:
	ebib.info