Fix overflowing banner repeating bug (issue #19)

If the banner spans larger than the width of the terminal, the banner
repeats on multiple lines. This commit changes
SpinningCursor::ConsoleHelpers.reset_line to clear overflowing lines and
move the cursor back to the starting line before printing again.

Also adds minitest gem to Gemfile because I needed it there to run tests
on my work machine.

This possibly breaks things for Windows (hah) due to more ANSI escape
sequence fun :).