Add transaction support on createJob, insert & fetch

[klesgidis]

Summary

Enables user to be able to pass a db interface that implements the executeSql function

With this changes users are now able to pass their own DB which enables them to add a job inside a transaction

[coveralls]

Coverage remained the same at 100.0% when pulling fb985ee3acbf731ef2fbd62ab4bedd0aeeb5359d on klesgidis:transactions into dbd8f981b955f53ae503cc9cac6d835fa7bae33d on timgit:master.

[timgit]

Thanks!