drop table works for tachyon table.
fix SHARK-165
move drop table tachyon related logic to unpersist in SharkEnv.sh
better logging
address comments
Merge pull request #112 from amplab/master

Merge master
update tachyon api
exclude Kyro from Tachyon
better logging
better logging
Merge branch 'master' into branch-0.8
remove hadoop from tachyon in sbt build
Merge branch 'master' into branch-0.8
better enable
better enable

no tachyon by default for now since tachyon 0.3.0 has not been pushed to public repo.
Merge branch 'branch-0.8' of github.com:amplab/shark into branch-0.8
Merge pull request #110 from amplab/branch-0.8

drop table works for tachyon table.