Disable echo of password in btsx client (Issue #29)

I added bool _no_echo_ argument to get_line with default value of false to make it compatible with single argument calls.
Please let me know if you want me to rename it to just _echo_ so it wouldn't sound double negative (no_echo = false), so it would be get_line( const std::string& prompt = ">>> ", bool echo = true ).

I've tested it on Os X and Linux - works fine, I can test a Windows version as well but it will take me a little while to setup Windows environment.