Add support for nested structure, e.g. attachments

[greut]

The optional attachments argument should contain a JSON-encoded array of attachments.

https://api.slack.com/methods/chat.postMessage

This enables simpler calls like this:

sc.api_call('chat.postMessage', {'attachments': [{'title': 'hello'}]})
# instead of
sc.api_call('chat.postMessage', {'attachments': json.dumps([{'title': 'hello'}]}))

[greut]

Damn... it took me one week to realize the bugs I've introduced. I was probably too tired to see anything.

[coveralls]

Coverage decreased (-0.002%) to 60.082% when pulling 6011119 on greut:nested into 87261f4 on slackhq:master.

[coveralls]

Coverage increased (+0.2%) to 60.246% when pulling eb57cb5 on greut:nested into 87261f4 on slackhq:master.

[coveralls]

Coverage increased (+0.3%) to 60.408% when pulling ebd1fd3 on greut:nested into 87261f4 on slackhq:master.

[coveralls]

Coverage increased (+0.5%) to 60.408% when pulling 13f4373 on greut:nested into 4fc299f on slackhq:master.

[moos3]

Any chance of getting this merged in?

[Yasumoto]

Mind adding six to requirements.txt as well?

[Yasumoto]

This is great! If you also don't mind updating the documentation to ensure other users know this exists, that'd be really helpful.

Normally I'd also ask you to add test coverage, but we don't have any good examples quite yet! If you're familiar with responses and don't mind adding test cases that'd be great, otherwise you can skip 'em for this PR and I'll add tests after this gets merged.

[coveralls]

Coverage increased (+1.3%) to 61.224% when pulling bcf0a5f on greut:nested into 4fc299f on slackhq:master.

[greut]

@Yasumoto sure, there you go. Travis seems to fail with coveralls.

As of responses, I care about what's send to Slack rather than having to deal with Slack's responses.

Cheers,

[coveralls]

Coverage increased (+1.3%) to 61.224% when pulling 5ab1954 on greut:nested into 4fc299f on slackhq:master.

[Yasumoto]

Thanks so much @greut !

[greut]

@Yasumoto my pleasure. Congrats on your new job at SlackHQ.