Use github math display in readme

[jishnub]

Also replace jldoctest blocks by juila to enable syntax highlighting. jldoctest is perhaps not necessary in the Readme file, as this isn't evaluated by Documenter.

Preview here

[codecov]

Codecov Report

Merging #415 (f492d5d) into master (d554649) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #415   +/-   ##
=======================================
  Coverage   83.69%   83.69%           
=======================================
  Files          23       23           
  Lines        2797     2797           
=======================================
  Hits         2341     2341           
  Misses        456      456           

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[jverzani]

Thanks!