obspy Stream plot wiggle ready

Simple wiggle plot based on Caparica's code.

```
    from obspy.segy.core import readSEGY
    %cd gngdata
    %cd OtherSegy
    cloudtraces = readSEGY("cloudpspin635.segy")
    from fatiando.vis import mpl
    mpl.wiggle(cloudtraces, normalize=True)
```

![wiggle](https://f.cloud.github.com/assets/4503573/1205990/f009b6e4-2597-11e3-88df-bb018e83f656.png)

don't kill me :skull: , is just a dataset i will not use it again I promise :laughing: