change the instrument pair format from 'aaabbb' to 'aaa_bbb'

[zhuyuecai]

the format of pair string is too hard coded. Thus the whole package can only deal with pure currency pairs. But for some new supported product, it doesn't work at all, ex:AU200_AUD, wheat_usd, etc. The format should be changed such that instead of
instrument = "%s_%s" % (event.instrument[:3], event.instrument[3:])
we can simply input the pair string. and we just need pair.split('_') to get the list of the two instrument.

moreover, why not use some oanda api wrapper to do the execution instead of writing our own?
There are well developed python wrappers for v1 and v20 account, we just need to do an conditional import