Pyfilesystem doesn't read ssh config

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Steps:
* Have an existing ssh config in ~/.ssh/config, referencing username and keys 
for different hosts. Have ssh keynames other than id_rsa or id_dsa.
* sftpfs will only find and use id_rsa or id_dsa. 

The enhancement here is that if there is an existing ssh config file, sftpfs 
would be able to find keys/configurations via this file. 

Original issue reported on code.google.com by orionrobots on 23 Sep 2013 at 7:24