Add config file for regtest/testnet

[ManfredKarrer]

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[ManfredKarrer]

@cbeams: I just checked in the config file support. I added a example file under docs. If it is available under the app data dir (/Users/username/Library/Application Support/Bitsquare/bitsquare.conf) it will override the network settings defined in the Guice module.

There is one open question how we handle the setup of 2 instances with passing the app name (that is used to separate the apps, otherwise they would access the same wallet and pref. files).
I use an parameter passed to the app (java -jar bitsquare.jar Alice) as app name.
With an native app we could pass it like that:
/Applications/Bitsquare.app/Contents/MacOS/bitsquare Alice
/Applications/Bitsquare_bob.app/Contents/MacOS/bitsquare Bob

Have not found a better way to do that yet. We cannot use the config file for the app name as we already need to have that information when loading the config (app name is used in the path).