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Auto merge of #272 - puremourning:tern-completer-locking, r=Valloric

[READY] JavaScript completer using Tern.js

# Overview

Adds support for a JavaScript completer using [Tern.js](http://ternjs.net).

The completer supports:

* Intelligent auto-completion
* Go to definition, references (`GoTo`, etc.)
* Semantic type information for identifiers (`GetType`)
* View documentation comments for identifiers (`GetDoc`)
* Management of `tern` server instance

This provides (for Vim at least) a replacement, for [tern_for_vim](https://github.com/ternjs/tern_for_vim) automagically in ycmd.

## Demo

Completions:

![ycm-tern-demo](https://cloud.githubusercontent.com/assets/10584846/11763113/035f748c-a0f6-11e5-8961-77139d4e6f45.gif)

Subcommands:

![ycm-tern-demo-subcommands](https://cloud.githubusercontent.com/assets/10584846/11763115/0523467c-a0f6-11e5-8552-357bcdda2e6a.gif)

## Installation

Run `install.py --tern-completer`

## Caveats and differences vs. tern_for_vim

* Currently, to use it there must be a `.tern-project` configuration file somewhere in the directory tree above Vim's working directory when the first JavaScript file is opened (i.e. when the server starts). I believe this is also true for `tern_for_vim`.
* Does not support refactoring (rename) which `tern_for_vim` does
* Methods from `Object.prototype` are always included. Tern has the ability to only include them when they match some number of typed characters, as they are otherwise "noise". This is incompatible with ycmd's cache and LCS matching.

## Status

I believe this to be fully working with the above caveats. I think the first one is the most frustrating, but it isn't obvious how to avoid it.

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