Date Format Issue with Data Providers.

[mwbrulhardt]

When trying to use the ExchangeDataProvider class, there is an exception when trying to use the next_ohlcv method. The issue is with giving the "since" parameter a string for the datetime. Instead of a string it wants the timestamp of that date passed as an integer.
This code produces an HTTPError.
`
import dateutil.parser

dp = ExchangeDataProvider()

since = '2018-01-01T00:00:00Z'
dp.exchange.fetchOHLCV(symbol=dp.symbol_pair, timeframe=dp.time_frame,
since=since, limit=1)
This code executes fine.
import dateutil.parser

dp = ExchangeDataProvider()

since = int(dateutil.parser.parse('2018-01-01T00:00:00Z').timestamp())
dp.exchange.fetchOHLCV(symbol=dp.symbol_pair, timeframe=dp.time_frame,
since=since, limit=1)
`