Send JSON as payload so that ampersands don't break messages

The Slack API specs say that you should either send `payload=...` as a POST
param or send JSON in the request body.  When sending the `payload` parameter,
Slack appears to do normal URI param parsing, treating ampersands as a param
separater even if they are buried in the JSON.

Sending the JSON as the body fixes this issue.
Merge pull request #1 from ewr/SendPayloadAsJSON

Send JSON as payload so that ampersands don't break messages
Merge branch 'master' into test_coverage

Conflicts:
	lib/slackr/webhook.rb
	spec/webhook_spec.rb
Adds changes from pull/1 that didn't make the merge