Fix KeyError exception

[jrfernandes]

Apparently, result is considered optional in some endpoint responses, such as getting program details:

ResponseOfProgramResponse {
    errors (Array[Error], optional),
    requestId (string, optional),
    result (Array[ProgramResponse], optional),
    success (boolean, optional),
    warnings (Array[string], optional)
}

http://developers.marketo.com/rest-api/endpoint-reference/asset-endpoint-reference/#!/Programs/getProgramByNameUsingGET

I did check a couple of endpoints in the documentation and this is not true for all of them, but, to be safe, this PR handles possible KeyError exceptions and returns an empty list as default.

[abendig]

Can you please add test coverage?

[jrfernandes]

Done.

[abendig]

Man, the test coverage in that package is at least as bad as the functionality provided ...

[abendig]

Changes look safe and good.

[jrfernandes]

Ahaha. True that. Might be useful, one day, to start our own cleaned library. Anyway, there's one last commit here to bump the package version in order for pip to install it in case the old version is already installed.

I'll probably open a PR for the original repo, eventually.

[abendig]

Would you mind adding this project to Slack (and ideally CI, though I don't know if that's easily possible)?

[jrfernandes]

I added this repo to our Slack integration. Regarding CI, I'll see about adding a free version of CircleCI here.