DataManager: Refactor code, put instance members at top of file
Put static instance members at the very top, and instance members
below, and constructor as first method in class.
CertsManager: Refactor, split into separate CertsDBHelper class
Put the DBHelper inner class into separate CertsDBHelper class,
which is like it is done for other DBHelper classes in the project.
AccountManager: Refactor, move db code into AccountDbHelper
Make access to database consistent with how it is done in other
DbHelper classes in the project.
Do not open and close the database for each method call.
Merge pull request #81 from alfh/fix_code_review_changes

DBHelpers: make code consistent, only open database once