async: remove racey `uv_ref()`/`uv_unref()`

`uv_ref()` can't be called from the non-event-loop thread. Furthermore,
it is not necessary in `async.h`, because open `uv_async_t` is already
holding the event loop from exiting.

See: #702

[skip travis] AppVeyor: use npm 2.15.9 for node > 0.x to work around "'__pfnDliNotifyHook2': redefinition;"

Appveyor node 0.10.40 fix

Fix interrupt test: wait for exec to complete before continuing with test

Add support for sqlite3_interrupt (with fixed test)

Revert "Add support for sqlite3_interrupt"

Add support for sqlite3_interrupt

  - Mysterious failures for now that @bergwerkgis will look into later

This reverts commit f1416bea.

Upgrade node-pre-gyp to support custom binary hosting mirror

https://github.com/mapbox/node-pre-gyp/pull/170

Add install help guide to contributing.md

- Users are prompted to view this guide before raising an issue. 
- Putting it here as well as in the wiki may help more people to see the info needed to debug their install.