ENH: escape=True option for to_latex #6472

Added an option escape=True to the to_latex() method (which is by default True) to be backwards
compatible, although setting this to False may actually match better the html behaviour when mathjax is used. It would be rather complicated though, to make to_latex always compatible with mathjax.

Also fixed a bug in the latex character escaping, as latex commands generally need to be terminated,
which was not the case so far and lead to latex compilation errors when the command was followed
by a normal character, as this character gets then consumed as the termination character.

Since I am new to git, github and nose I apologise for not separating the bugfix from the enhancement,
in the hope this is still better than nothing.