Add html table output to reporttools

It turns out that we would like to use reporttools tables in html documents. Although this pull request basically rewrites the tableContinuous, tableNominal and tableData functions and man pages, the actual changes are simple.

* add a type argument to the function calls to switch between "latex" and "html", 
* turn off longtable when using html.

I've also added your summary to a readme.md file for github, and included a Markdown vignette skelaton to display the output in html documents.