Add new embed endpoint support

[brettlangdon]

http://docs.datadoghq.com/api/#embeds

[brettlangdon]

partial support added from #31 to create graph embeds

[brettlangdon]

As of #34 and version 2.2.0 we have almost full support for embeds (thanks to @jkurz!!!).

The only endpoint we are still missing for full support is embed enable, GET api/v1/graph/embed/:embed_id/enable