Corrected the usage of font style highlighting

`\em` and `\tt` are TeX commands and need to be treated differently:
  {\em text} and {\tt text}. LaTeX you would use `\emph{text}` and
  `\texttt{}` without funny side effects for the following text.

In order to make it easier to quote commands I simply defined the
command `\code` which is just an alias for `\texttt` but nicer I think.