start to add stream support

first draft version with SDF support

returns vanilla stream by default, respects the order of data, url and path, returns streams in objectMode only if requested as option, support url using mikeal/request

add coerce option (coerce values according to their type in schema)

rename stream to createReadStream, add ldjsonify option (to return ldjson) and make foreignkey support optional even in objectMode
add support for inline data as strings and resource.format ldjson (file.ldjson)
Merge pull request #5 from standard-analytics/streams

[#5,stream][m]: support for streams - fixes #5.