The default fontspec font features are
```
\defaultfontfeatures
[\rmfamily,\sffamily]
{Ligatures=TeX}
\defaultfontfeatures
[\ttfamily]
{WordSpace={1,0,0},
PunctuationSpace=WordSpace}
```
There isn't a reason to override this (see PR #154).
beamer font definitions are now standard latex based
implemented conditional font loading scheme
added demo frame to test font features
better implementation of monospaced numbers in tables
