refresh portfolio stats every 11 minutes

some users would prefer the cex.io portfolio balance should be refreshed periodically with more current BTC balance (as a result of BTC withdraw, deposit, mining new coin with the GHS, etc.)

this commit will affect all exchanges, not just cex.io

refresh period is 20 minutes
actually refresh the portfolio before displaying balance

bugfix for gekko having a stale internal state which doesn't match the actual balance in portfolio
Merge pull request #67 from kuzetsa/refresh-balance

Refresh & log updated portfolio balance so that advice & trading is not based on old data
typo in auth checker
only refresh portfolio when using cex.io
removed code duplication
updated exchanges explenation
Merge pull request #85 from streetlogics/master

Fix for issue #72
reverted false bugfix (see pull/85)
Merge remote-tracking branch 'upstream/master'