Move JS assets into turbograft folder

Fixes #24 

Moved all JS assets under /turbograft folder. The manifest file now includes all files under the turbograft folder instead of individually including .coffee files in the same folder as the manifest. This should prevent any file name conflicts.

Review @qq99 @tylermercier 
The only place in initializers that would potentially be a problem is:
https://github.com/Shopify/turbograft/blob/9df7047e83af7212953d78014657a0052d29a7d6/lib/assets/javascripts/initializers.coffee#L7

But it's OK because `require_self` happens first and handlers is defined below.

If this becomes an issue down the road because of code we add into initializers then we could go back to requiring files in a specific order. But right now I could not find any hard dependencies between files.