DOC: Fix tutorial import. Closes #430.
BUG: Fix price caching for tickers with '/' char

On Ubuntu (assume this is true for all posix) tickers containing a slash char ("CRD/A", "BRK/A", both valid tickers with yahoo api accessible timeseries) lead to a path error in loader.py line 286.
ENH: Makes the offset of `market_close` relative to 20:00 UTC

Changed from relative to 20:01 UTC.