Add more configuration optoins in application.conf

Currently we only provide very minimum configuration options on application.conf, which can be found at target/apache-s2graph-$version-incubating-bin/conf/.

db.default.url="jdbc:h2:file:./var/metastore;MODE=MYSQL"
db.default.user="sa"
db.default.password="sa"

I think it would be better to provide configuration options with simple explanation on application.conf.

We have various configurations such as connecting remote HBase cluster and Asynchbase client configurations, S2Graph's cache configuration etc.

I am suggesting to list these reasonably important configurations with comment with informations.