Added set theory and logic symbols

\subset, \supset, \subseteq, \supseteq, \cap, \cup, \setminus

\neg, \lnot, \top, \bot, \emptyset, \varnothing, \land, \lor,
\wedge, \vee, \notin, \nsubseteq, \nsupseteq, \models

Did not add \forall, \exists and arrows as I see there are already pull requests for those.

Resolves #52 and #62 ( with pull request #58 )