Change latex for ' ' from '\\:' to '\\ '

Writing a backslash (which triggers LatexCommandInput) and then hitting
Space, Tab, or Enter used to insert the backslash character when the
latex of the left end was empty when really it should have inserted a
space.

We changed `CharCmds[' ']` to a `LatexCmds[' ']` since latexMathParser
can parse `\ ` fine and `LatexCmds` only checks for the control
sequence.
Test LaTeX parsing of `\ `
Alias `\space` to `\ `

In addition to being used in various forks of MathQuill (such as
Desmos's), which we'd like to remain back-compat with, it's a real TeX
command that means something pretty similar to `\ `:
http://tex.stackexchange.com/questions/119717/usage-of-latex-macro-space-compared-with
Merge pull request #268 from mathquill/fix.latex-space

Change latex for ' ' from '\:' to '\ '