Resubmitting - because GIT.

This is the last cut that was in #1117 before i closed that PR

This PR allows a user to set the flag "ta_on_candle" in their config.json

This will change the behaviour of the the bot to only process indicators
when there is a new candle to be processed for that pair.

The test is made up of "last dataframe row date + pair" is different to
last_seen OR  ta_on_candle is not True

freqtrade/constants.py

@@ -53,6 +53,7 @@ CONF_SCHEMA = {
         },
         'fiat_display_currency': {'type': 'string', 'enum': SUPPORTED_FIAT},
         'dry_run': {'type': 'boolean'},
+        'ta_on_candle': {'type': 'boolean'},
         'minimal_roi': {
             'type': 'object',
             'patternProperties': {