org-element: Make properties parsing more robust

* lisp/org-element.el (org-element-property-drawer-parser,
  org-element-node-property-parser): Ignore lines that are not node
  properties.
(org-element-node-property-interpreter): Allow nil properties.

* lisp/org.el (org-re-property): Fix regexp to match properties with
  empty values.

* testing/lisp/test-org-element.el (test-org-element/node-property):
  Add tests.

Thanks to Eike for reporting it.
http://permalink.gmane.org/gmane.emacs.orgmode/90293
Fix 37bf05

* lisp/org.el (org-re-property): Fix regexp.  Improve docstring.
* testing/lisp/test-org-element.el (test-org-element/node-property):
  Update test.

Thanks to Daimrod for reporting it.
http://permalink.gmane.org/gmane.emacs.orgmode/90383
org.texi: Fix Texinfo export documentation
orgguide.texi: Small fixes

* doc/orgguide.texi (Export options): Small fixes.

Thanks to Alexis Roda for reporting them.
org.texi: Tiny fix
Get the part finding an attachment out of `org-attach-open'.

* lisp/org-attach.el (org-attach-find-file): Created
* lisp/org-attach.el (org-attach-open): Make use of `org-attach-open'

This will allow other functions to make use of the `org-attach-find-file'
feature.
Add a support for linking against an attachment of current entry.

* lisp/org-attach.el (org-attach-open-link, org-attach-complete-link): Created

Add a call to `org-add-link-type' to make use of the feature.