Move the 'pretty' bin to 'trees' example bin

Docopt requires bin and library have different names. We can have multiple
binaries (the [[…]] stands for an array entry) so if we need to add something
else later it should be fine.
Remove unnecessary pragmas
Sort derives
Small formatting tweaks
Vec should generally be passed by-value

See https://mail.mozilla.org/pipermail/rust-dev/2014-June/010094.html
Pass self by-value to avoid explicit clones
Use Str trait
Use ToString trait to avoid separate funs
Readability tweaks
'left' should always be >= 0 as uint
Clone at the match instead of the arms
Use vec! notation
Use iterator to repeat elem
Use mutable arg
Merge pull request #9 from darinmorrison/master

Cleanup; simplification; etc.