Implement con_find_named method

The find_named method will search descendent nodes with names that match
the given search pattern and return the result as a list of containers.
Implement con_leaves method

 The con_leaves method finds the leaf descendent nodes of a given
container excluding dock clients.
Add con window_class property

The window_class property of the con is the class of the window
according to WM_CLASS as reported by the ipc.
Implement the con_find_classed method

The con_find_classed method finds a list of descendent Cons which have a
WM_CLASS class property as reported by the ipc that matches the given
pattern.
Bugfix: keep a reference on the parent con

Without keeping a reference on the parent con, the tree can lose its
integrity when the parent con falls out of scope such as when the child
is within a seperate container.
Add install location hint to README
Bump to version 0.3.0