updated some documents
Added user guide
Fix the bug of coverage to close to 1

Merge branch 'master' of https://github.com/JohnsonHsieh/inext

Conflicts:
	server.R
	ui.R
	www/about.md
	www/footnote.md
bug fix for download csv file
1. bug fix for slider bar
2. remove default data set
3. improve computing efficiency