Add a debug flag to know what is happening

[eloyesp]

As mentioned in #294 it just adds a debug flag to see what commands are being run.

I don't know how to add tests for it :(

[havenwood]

On the testing front, it doesn't address the feature functionality, but it'd be nice to test the option parsing in test/ruby-install-tests/parse_options_test.sh.

[eloyesp]

@havenwood I did that but it adds a xtrace for shunit with a lot of noise within the tests :)

function test_parse_options_with_debug()
{
       parse_options "--debug" "ruby"

       assertTrue "did not set -x within the script" "[ -o xtrace ]"
}

[havenwood]

@eloyesp Ahh, good point.

[postmodern]

My only concern is that set -x may be too verbose for most use cases.