Re-open panel at last position after deletion

Doesn't work correctly currently because of SublimeText/Issues#222 but
this is the best we can do.

Closes #24.
Catch exception when loading history file fails

Fixes #27.
Add setting for re-include path patterns

Closes #22.
Update a few comments in the settings file
Revise README and delete settings sections

We have the settings explained in the settings file itself and also a
small overview of available settings in the Features section.

Also move one screenshot to the top because that way users can more
quickly see what this plugin does.
Some minor refactoring wrt cleaning up callbacks