DashDoc and Dash v1.9.3

I have adapted DashDoc to follow Dash's (v1.9.3) new URL scheme.  

Other changes:

- Incoporated Dash's proposed default syntax-to-docset mapping
- Individual syntax-to-docset mappings can now be overridden in the `settings` of a project's `sublime-project` file 
- Syntax sensitivity is the default now

Let me know what you think.  Cheers and a merry christmas,
  —Torsten