Add transaction support

[kipz]

• Taken from #123 - thanks to @swvist
• Resolves #122

[kipz]

@joelittlejohn @neilprosser it's worth noting that transact-write-item now always returns a map to be consistent with the batch operations.

e.g. https://github.com/Taoensso/faraday/pull/145/commits/ef5f1f04522169f0d78371698f589ac537589eb1#diff-eb45e1f169385fa1d90ecb43a6665568R1418

[nxvipin]

Thank you!