Added launching by configuration file.

I've added two functions to read a configuration file and launch based upon that.

ReadConfiguration reads a configuration file and configures various globals.
An example file would look like

ClientTransportPlugin obfs3 obfsproxy managed
ClientTransportPlugin flashproxy flashproxy-client --transport "obfs3|flashproxy" 127.0.0.1:0 :9000
Alias obfs3_flashproxy obfs3|flashproxy

The other function I added was a launch_pt_chain function. It takes the chain described in the configuration file and launches each pluggable transport in turn. Currently it only works with two pluggable transports, but it will be easy to adapt it to running any number.