Custom timestamp for every value

Specify timestamp for each value in send_dict() and send_list()
Then data in send_list will be three-tuple:
(metric, value, timestamp)

Adding this feature in send_list() should be pretty easy. A second data
structure would just need to be supported.

From:
[ (metric, value), (metric, value) ]

To:
[ (metric, value, timestamp), (metric, value, timestamp) ]

This would support multiple value-timestamps per metric and multiple
metrics per value-timestamps (WIN-WIN)
Removed graphitesend from requirements
Added send_list() docs to README