Fixing the EJS context

I've made tweaks to how the rendering context works. I fixed several bugs and made it more consistent. From the USAGE.md docs: 

## Render EJS templates

Lingon comes with out of the box support for EJS templates using the [gulp-ejs](https://github.com/rogeriopvl/gulp-ejs) module. 

### Render data with EJS

The Lingon instance has a property `lingon.global` which is available in all EJS templates during rendering. The object can be accessed from all EJS templates & layouts.

**Example: lingon.js**

```js
var lingon = require('lingon');

lingon.global.name = "bob";
```

**Example: index.ejs**

```html
<html>
  <%= global.name %>
</html>
```

### File specific context

EJS templates also have access to a `context` object that contains information about the current file being processed.

Name | Description
-----|------------
file | *Path to current file being rendered*
layout | *Path to current layout (null if no layout is used)*
template | *Path to the current source file**

**"source file" refers to the file being written to the build path after partials and layouts have been included.*

The `context` object can also be extended with custom properties in a pre-processor. For example, use this to get the shasum of each file and write it in the template.

**Example: lingon.js**
```js
var lingon = require('lingon');
var es = require('event-stream');
var spawn  = require('child_process').spawn;

lingon.preProcessors.unshift('ejs', function(global, context) {
  return es.map(function(file, cb) {
      var shasum = spawn('shasum', [
        context.file
      ]);

      shasum.stdout.on('data', function (data) {
        context.shasum = data.toString().trim();
      });

      shasum.on('close', function (data) {
        cb(null, file);
      });
    });
});
```

**Example: home.ejs**
```html
<div class="home">
  <h1>file: <%= context.file %></h1>
  <h2>layout: <%= context.layout %></h2>
  <h3>template: <%= context.template %></h3>
  <h4>metadata: <%= context.shasum %></h4>
</div>
```