Added various 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
Merge pull request #72 from evestera/setrelops

Added set theory and logic symbols