Revert "Imported github.com/peterh/liner lib."

This reverts commit 6ffaae08dfb8d53729289059a849107a75fce478.

Un-vendoring this package, as go6502 is moving to a more standard Go
directory layout as described at http://golang.org/doc/code.html
File/dir layout as per http://golang.org/doc/code.html

* Now `go get github.com/pda/go6502` should work.
* This repo should be at `$GOPATH/src/github.com/pda/go6502`.
* The built binary ends up in `$GOPATH/bin`
* `rom/kernal.rom` remains relative to the current (invoking) directory.