Downgrade Java 1.7 to 1.6.
Merge pull request #3 from hyunsik/DOWNGRADE_JVM

Downgrade Java 1.7 to 1.6.
Replace Objects and BitSet of JAVA 7 with substitutes.
Merge branch 'master' of https://github.com/tajo-project/tajo into UPDATE_SITE
Add 'Why Tajo?' section and a slide link, and update the introduction.