Plotting for offset-sections added.

Plotting of record sections added.

Offset is either read in meters from ```tr.stats.distance``` or azimuthal distance calculated from ```trace.stats.coordinates.``` with passed events latitude and longitude.

Traces can be scaled ```scale=1.0``` and a velocity reduction can be performed ```vred=6500```.

```python
st.plot(type='section', plot_dx=10e3, outfile='sample_section.png')
```
![Record Section](https://f.cloud.github.com/assets/4992805/888826/7953ff7c-fa14-11e2-8b3d-16960000d61a.png)

complete list of parameters:
```
        :type scale: scalar, optional
        :param scale: Scale the traces width with this factor.
            Default is to ``1.0``
        :type vred: scalar, optional
        :param vred: Perform velocity reduction, in m/s.
        :type norm: 'stream', 'trace', optional
        :param norm: Defines how the traces are normalized,
            either against each trace or against the global
            maximum. Default is ``trace``
        :type offset_min: Scalar or None, optional
        :param offset_min: Minimum offset in Meters to plot.
            Default is minimum offset.
        :type offset_max: Scalar or None, optional
        :param offset_min: Maximum offset in Meters to plot.
            Default is maximum offset.
        :type azim_dist: bool, optional
        :param azim_dist: Plot in azimuthal distance from event.
        :type ev_lat: float or None, optional
        :param ev_lat: Events' latitude in degree.
        :type ev_lon: float or None, optional
        :param ev_lon: Events' longitude in degree.
        :type plot_dx: Integer, optional
        :param plot_dx: Spacing of ticks on the spatial x-axis.
            Either km or degree, depending on ``azim_dist``
        :type recordstart: Integer, optional
        :param recordstart: Seconds to crop from the beginning.
        :type recordlength: Integer, optional
        :param recordlength: Length of the record section in seconds.
        :type alpha: Float < 1.0, optional
        :param alpha: Transparancy of the traces between 0.0 - 1.0.
        :type time_down: Bool, optional
        :param time_down: Flip the plot horizontaly, time goes down.
            Default is time goes up.
```

Thankful for comments and tips since this is my first commit.

cheers, Marius

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Ran 14 tests in 26.488s

OK