Added Plotting of offset-sections added.

Ran 14 tests in 26.488s

OK

Sample record section.
![Record Section](https://f.cloud.github.com/assets/4992805/888826/7953ff7c-fa14-11e2-8b3d-16960000d61a.png)

st.plot(type='section', plot_dx=10e3, outfile='sample_section.png')

Thankful for comments and tips since this is my first commit.

cheers, Marius