Refactored smells out of no_brainer/database.rb

Smells reported by reek:

Before:

 1: NoBrainer::Database has no descriptive comment
    (IrresponsibleModule)
20: NoBrainer::Database#purge! contains iterators nested 2 deep
    (NestedIterators)
24: NoBrainer::Database#purge! has the variable name 'e'
    (UncommunicativeVariableName)

After:

 1: NoBrainer::Database has no descriptive comment
    (IrresponsibleModule)

Notes:

Extracted self.truncate_table! utility function and made variables
more communicative.