Add valid bar sizes from IB

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• 10 secs
• 10,20 mins
• 2,3,4,8 hours
• 1 week
• 1 month

Although when an invalid bar size is used, the returned error message uses 1W and 1M, I have tested that both '1 week' and '1 month' work.

Reference: http://interactivebrokers.github.io/tws-api/historical_bars.html#gsc.tab=0

Note: Append "?w=1" in the diff url to ignore whitespace. (eg. https://github.com/gofinance/ib/pull/30/commits/660ffe77a56fc70f0ab92052907cda624f173bb9?w=1)

[nothize]

@creack @indrekj @joelreymont @uwe

Please review, thanks!