Just using $ as delimiter in markdown document fails since the markdown
parser unknown to the dollar syntax will try to interpret underscores.
Putting the $ delimented formula in backticks will cause the markdown
parser to put the tex-code with the $ delimiters into a code block.
The texcode will then be unchanged. This patch allows for mathJax to
interpret and automagically display the tex-formulas.

by changing the tag before html is parsed and restoring it before markdown parsing

Compressed with `uglifyjs lib/marked.js --comments="/Copyright/" --mangle --compress`

Used Marked v0.3.3 as of chjj/marked@2b5802f258c5e23e48366f2377fbb4c807f47658

see https://github.com/hakimel/reveal.js/issues/929#issuecomment-80738837

add global flag back in for whitespace removal, without it most of the content in example.html does not work #682

fixed two minor bugs, a twohanded gesture may not have a direction, and if autoCenter is of, the tipPosition was in the wrong scope

renamed markdown attributes for clarity and consistency: data-vertical -> data-separator-vertical, data-notes -> data-separator-notes