Remove invalid references due to syntax

[jayvdb]

Github is creating links to non-existant resources due to the presence of [..] (..) syntax. Github allows a space between these components, while the space avoids links in other markdown parsers.

These mostly occurred because [..] is being used to denote subscript within equations.

Replace these equations with LaTeX, and add \hspace{0pt} between any literal square brackets and round brackets to prevent links.

Also add missing sum in equations for problem 444, and remove a stray ^ from problem 461.

Fixes #3

[jayvdb]

gah; this patch (75f5a6a4) didnt include the fix for problem 186 , but it passed the link validator. :/

[imsky]

:+1:

[jayvdb]

Travis not running atm; https://github.com/travis-ci/travis-ci/issues/6292