Patch SFTPFile to be used as an context manager.

paramiko.SFTPClient.open returns a file-like object SFTPFile. SSHConnect
returns an instance of SFTPFile but first patches the class to implement
the context manager protocol (enter and exit methods).
Support for global config (at ~/.pic/config).

All command line options can now be globally configured, but command line flags take precedence.

Examples:

[viewer]
prog = gthumb

[backup]
path = /mnt/backup/photos:/home/user/photos
Updating README, requirements & backlog

README converted to markdown syntax
new requirements.txt file to be used as input for pip
added python3 work item to backlog