Added syntax sensitivity to DashDoc

Hi Vladimir,

I've been using your DashDoc package for Sublime Text 2 for quite some time now and I love it!

With this commit I have added syntax sensitivity to DashDoc: users have the option to let Dash search only the docset that is relevant to the syntax of the current view (looking up `map` means something different for Haskell and Python buffers).

I tried to be non-invasive: syntax sensitivity is off by default and DashDoc's key binding `Ctrl-h` has not been changed.  I have edited the README to reflect the changes and listed the new option.  I have also added menu entries under ST2's *Preferences* menu and added DashDoc to the editor's command palette.

Let me know what you think.  

Cheers and greetings from Germany,
   —Torsten

- DashDoc can now instruct Dash to consult a a docset specific to the syntax of the current view (default: off; a mapping between syntax and docset can be provided)
- Added key binding Ctrl-Alt-H to enable syntax sensitivity in a one-off fashion
- Added DashDoc settings file
- Added DashDoc entry under ST2's preferences menu
- Added DashDoc commands to ST2' command palette
- Adapted the README