Add JSON support

[ivotron]

Aver assumes that metrics are stored in a relational table that is accessed using database/sql. In order to support JSON, we need to find a way to load the data into a table. We can obtain a CSV file from JSON that then gets imported into a SQL backend. The CSV-to-table conversion is an independent task that is covered in issue #3. This issue addresses going from JSON to CSV.

It is relatively straight-forward to go from JSON to CSV (e.g. using json2csv), the problem is that given the unstructured nature of JSON, there's no single way to interpret a JSON-formatted file. Thus, we need to create an importer for each distinct metrics backend. This issue will cover some of the most popular open source ones:

• graphite
• influxdb
• opentsdb

Others not covered by this issue: ganglia, librato, stathat, ...