Initialize the quick help plugin even if the plugin fails to swizzle a method.

The other swizzling might succeed.
Open all doc links in Dash.

This adds the ability to intercept option-double-clicks on symbols,
as well as to open "quick help autocomplete" links.

This feature can be disabled, like the rest of Dash, by selecting the "Disabled"
menu item within the "Dash Integration" submenu of Xcode's "Edit" menu.
Merge pull request #18 from wearhere/jeff/open_all_doc_links

Open all doc links in Dash.
Use the new dash-plugin:// URL scheme

- Platform detection just got a whole lot better as it now uses the new
dash-plugin:// URL scheme introduced in Dash 1.9.3. This URL scheme is
documented at http://kapeli.com/dash_plugins.
- Platform detection is now able to recognise C, C++, Objective-C and
Objective-C++ files and enable the appropriate docsets for them.