Org reader: parse LaTeX-style MathML entities

Org supports special symbols which can be included using LaTeX syntax,
but are actually MathML entities.  Examples for this are
`\nbsp` (non-breaking space), `\Aacute` (the letter A with accent acute)
or `\copy` (the copyright sign ©).

This fixes #1657.
Merge pull request #1706 from tarleb/org-symbol-entities

Org reader: parse LaTeX-style MathML entities