Refactor --initdb --upgradedb arguments (IDR-0.4.0)

[manics]

Follow-on from #103

Replaces the separate init_db upgrade_db methods in omego {install,upgrade}with a singlehandle_database` method that deals with the logic required for handling intersecting flags.

Also adds -v ON_ERROR_STOP=on to all psql calls

Proposed tag: 0.6.1 (0.6.2 to follow with gh-95)

[joshmoore]

@manics : did you look into testing the return codes in this PR?

[manics]

I didn't

[joshmoore]

I'm not successfully getting the exit codes via $? that you've shown here. I'm merging this as 0.6.1 to allow Seb's work to continue and have filed #106 so we can come back to tests for this.