opentxs: bash autocompletion for commands

Add autocompletion for opentxs commands. E.g.:
opentxs a (press tab output:)
lucas@linux-nouu:~/Projects/OpenTransactions> opentxs a
acceptall        acceptinbox      acceptinvoices   acceptmoney      acceptpayments   acceptreceipts   accepttransfers  addasset         addserver        addsignature  

+ Updated Linux docs

This will work on Linux, maybe also OSX and other Unix system, but not for Windows.
Since it seems like the path for the file is different on various Linux system, randy-waterhouse has added a --with-bashcompletion config flag.

Second commit is because I had to merge his repo I copied to my HD with upstream repo.