Automagically convert files in $GOPATH/data

Hi @jteeuwen!

I've just had too much time I thought I'd throw some magic at your `go-bindata`. And by coincidence it actually happened that I had pretty solid use-case to leverage this functionality. If you'd happen to like it, it'd be awesome if you'd merged it with your project. Below I paste some description:

### Automagic conversion within `$GOPATH` workspace

When no input files nor directories are provided via command line flags,
go-bindata reads `$GOPATH` workspaces and attempts to convert all files it
finds recursively in `$GOPATH/data`, generating a `bindata.go` file in a matching
`$GOPATH/src` directory. The match is always the longest path diff between
`$GOPATH/data` and `$GOPATH/src`, in order to avoid having assets which content
overlaps. For example, running:

```bash
  ~ $ GOPATH=/home/user go-bindata
```

over the following $GOPATH workspace:

```bash
  /home/user
  ├── data
  │   ├── bitbucket.org
  │   │   └── user
  │   │       └── hidden
  │   │           └── subpackage
  │   │               ├── aws.txt
  │   │               └── pass.txt
  │   └── github.com
  │       └── user
  │           └── example
  │               └── assets
  │                   ├── css
  │                   │   └── default.css
  │                   └── js
  │                       ├── app.js
  │                       └── link.js
  └── src
      ├── bitbucket.org
      │   └── user
      │       └── hidden
      │           ├── hidden.go
      │           └── subpackage
      │               └── subpackage.go
      └── github.com
          └── user
              └── example
                  └── example.go
```

will create two asset files under the following paths:

```bash
  ~ $ GOPATH=/home/user go-bindata
  ok      bitbucket.org/user/hidden/subpackage    (/home/user/src/bitbucket.org/user/hidden/subpackage/bindata.go)       0.001s
  ok      github.com/user/example (/home/user/src/github.com/user/example/bindata.go)    0.002s
```

Running go-bindata in this mode will ignore any values passed by `-o`, `-pkg` and
`-prefix` flags.