Fixed recipe 6 in chapter 6.
Updated recipe 4 in chapter 6 with NetworkX export function.
Updated discount code.
Adding WIP installation instructions.
Updated instructions.
Updated recipe 1.1.
Updated recipe 1.2.
Updated recipes 1.3 and 1.4.
Updated recipe 1.5.
Updated recipe 1.6.
Replaced old %%file magic by %%writefile.
Fixed bugs in recipe 3.2.
Updated recipe 3.3.
Minor style fix in recipe 3.6.
Added py2x and py3x versions of recipe 3.7.
Fixed recipe 3.7.
Fixed links to recipe 3.7.
Updated link to @jasongrout's example.
Updated recipe 4.3.
Updated recipes in chapter 4.