Replaced relative import paths to enhance usability on github.

Hi,

at first I want to thank you guys for creating this amazing toolchain! 

I made this change because I tried to use zgrab and ran into the following error:
```
$ go get github.com/zmap/zgrab
package github.com/zmap/zgrab
        imports zgrab/zlib: unrecognized import path "zgrab/zlib"
package github.com/zmap/zgrab
        imports ztools/processing: unrecognized import path "ztools/processing"
package github.com/zmap/zgrab
        imports ztools/zlog: unrecognized import path "ztools/zlog"
package github.com/zmap/zgrab
        imports ztools/ztls: unrecognized import path "ztools/ztls"
```

Then I read the README (that's what I should have done first ;)) and did the steps to successfully build zgrab, which was more complicated than a usual go-build for github projects.

```
$ cd $GOPATH/src
$ git clone https://github.com/zmap/zgrab.git
$ git clone https://github.com/zmap/ztools.git
$ cd zgrab && go build
```

So, with this change, you can remove the following line from the README:
```
Unfortunately, since zgrab is still actively under development, you will also need to clone the ztools repository to $GOPATH/src/ztools.
```
since it's not necessary anymore. With a `go get github.com/zmap/zgrab` all needed tools go to `$GOPATH/src/github.com/zmap/*`.

If there is another reason for local imports, which I'm not aware of, just ignore this pull request.

Regards,
Malte