Set random story link href after each click

This commit includes following changes:

- wrap initialization into Sammy init function
This is because of a problem: Handlebars base template
being added to DOM after Sammy tries to access its elements
when binding event handlers to header and stuff.

- move header initialization (bindings and other) to helpers.header.init()
Just to clean up code.

- generate href for random story link each time clicked
This link actually never refers to /random, it refers directly to the next
random story.
Link changes that story number every time accessed.