Add usage example and description to README.md

INSTALL.md was also updated to add download instructions.
Add links to sample data
Add introduction text
Merge branch 'master' of git://github.com/qiime/emperor into readme
Add updated versions of the GUI for all examples
Remove unnecessary files and update README file
Merge pull request #79 from ElDeveloper/readme

Add usage examples and description to README.md
Fixes repeated link in README.md
Merge pull request #81 from ElDeveloper/repeated_link

Fixes repeated link in README.md