fs/fsutil: Compact tree literals
fs/fsutil: Rework TestIntersect
fs/fsutil: Add a test-case for TestIntersect
fs/memfs: Add IsDir function
cmd/gotree: Do not count root directory
cmd/gotree: Support for -a flag
cmd/gotree: Support for -d flag
cmd/gotree: Don't print $PWD
fs/memfs: Refactor tree parsing out to TreeBuilder struct

Breaks public interface, the following gofmt commands takes care of 4/5
rewrites one must perform to fix ones code:

	gofmt -w -r 'TabTree -> UnmarshalTab' .
	gofmt -w -r 'Unix.Tree -> Unix.Decode' .
	gofmt -w -r 'UnixTree -> UnmarshalUnix' .
	gofmt -w -r 'ErrCustomTree -> ErrTreeBuilder' .

The remaining one is to change CustomTree to TreeBuilder, like in the
following example:

	- var ct CustomTree = func...
	+ var tb TreeBuilder = TreeBuilder{DecodeLine: func...
fs/memfs: Extend TreeBuilder with an Encode method
fs/memfs: Make FS implement Text{Unm,M}arshaler interfaces
cmd/gotree: Add -go flag for printing tree as Go literal
cmd/gotree: Refactor misc readers to separate tools/rw package
Update README.md
Update README.md
Update README.md
Reconfigure Travis CI to build under Mac OS X