Update release 0.7 examples' system

Initially this PR tried to make the experience of writing and reading examples better in Dipy. But now it is doing many more things in preparation for the shortly upcoming release. The most important are:

a) updates installation instructions
b) introduces many new examples
c) allows specific examples to be rendered in the website by commenting or uncommenting a file called valid_examples.txt in the examples folder.
d) in sims.voxel corrects the signature of multi_tensor_odf in agreement with   
e) corrects a bug in peaks_from_model concerning 0 sized peaks

This PR should preferably be merged after Mauro's (#257) and Bago's (#250) PRs are merged. So,that I can add their examples in the list of available examples and resolve some conflicts easier.

Many files have been slightly modified in this PR as it can be seen here
https://github.com/nipy/dipy/pull/279/files

Because there are many files that are changed I would suggest to concentrate first on checking a) the installation instructions and b) the new examples.