Refactored sql/bash execution functions and removed warnings

Refactored functions:

- Moved JDBC-related execute* functions into JdbcDataStore, so that all
ds can call it.
- Moved the function to execute SQL query via bash file into Helpers
- Bash command execution everywhere should call Helpers.executeCmd.

Removed warnings: Fixed warnings by importing scala.language.postfixOps
and scala.language.implicitConversions in corresponding places. DeepDive
can run without warnings now.