preserve Aspect Ratio for images support

This is draft of implementation of preserveAspectRatio attribute support for images, started to fix issue #384
Basically it can be:
Do not respect aspect ratio
Respect aspect ratio shrinking image to fit (meet)
Respect aspect ratio cutting the image. (slice)

those are the specs:
http://www.w3.org/TR/SVG/coords.html#PreserveAspectRatioAttribute

This is how it should look:
![image](https://cloud.githubusercontent.com/assets/1194048/4429185/2f09068a-45de-11e4-8483-cdc63b9e55b0.png)

I made the image transparent to show rectangles under it for reference.
This is how it looks as of now:
![image](https://cloud.githubusercontent.com/assets/1194048/4429173/d61e97e2-45dd-11e4-960d-f6ad9b33fdba.png)

this is the svg i crafted to show the example:
```xml
<?xml version="1.0" encoding="UTF-8"?>
  <svg width="450px" height="300px" version="1.1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" >
  <desc>Example PreserveAspectRatio - illustrates preserveAspectRatio attribute</desc>
  <rect x="1" y="1" width="448" height="298"
        fill="none" stroke="blue"/>
  <g font-size="9">
    <text x="10" y="30">SVG to fit</text>
    <g transform="translate(20,40)"><image xlink:href="http://www.deltalink.it/andreab/fabric/assets/smile.svg" width="30" height="40" ></image></g>
    <text x="10" y="110">Viewport 1</text>
    <g transform="translate(10,120)"><rect x='.5' y='.5' width='49' height='29' fill='none' stroke='blue'/></g>
    <text x="10" y="180">Viewport 2</text>
    <g transform="translate(20,190)"><rect x='.5' y='.5' width='29' height='59' fill='none' stroke='blue'/></g>

    <g id="meet-group-1" transform="translate(100, 60)">
      <text x="0" y="-30">--------------- meet ---------------</text>
      <g><text y="-10">xMin*</text><rect x='.5' y='.5' width='49' height='29' fill='none' stroke='blue'/>
        <image xlink:href="http://www.deltalink.it/andreab/fabric/assets/smile.svg" preserveAspectRatio="xMinYMin meet" width="50" height="30"></image></g>
      <g transform="translate(70,0)"><text y="-10">xMid*</text><rect x='.5' y='.5' width='49' height='29' fill='none' stroke='blue'/>
        <image xlink:href="http://www.deltalink.it/andreab/fabric/assets/smile.svg" preserveAspectRatio="xMidYMid meet" 
             width="50" height="30"></image></g>
      <g transform="translate(0,70)"><text y="-10">xMax*</text><rect x='.5' y='.5' width='49' height='29' fill='none' stroke='blue'/>
        <image xlink:href="http://www.deltalink.it/andreab/fabric/assets/smile.svg" preserveAspectRatio="xMaxYMax meet" 
             width="50" height="30"></image></g>
    </g>

    <g id="meet-group-2" transform="translate(250, 60)">
      <text x="0" y="-30">---------- meet ----------</text>
      <g><text y="-10">*YMin</text><rect x='.5' y='.5' width='29' height='59' fill='none' stroke='blue'/>
        <image xlink:href="http://www.deltalink.it/andreab/fabric/assets/smile.svg" preserveAspectRatio="xMinYMin meet" 
             width="30" height="60"></image></g>
      <g transform="translate(50, 0)"><text y="-10">*YMid</text><rect x='.5' y='.5' width='29' height='59' fill='none' stroke='blue'/>
        <image xlink:href="http://www.deltalink.it/andreab/fabric/assets/smile.svg" preserveAspectRatio="xMidYMid meet" 
             width="30" height="60"></image></g>
      <g transform="translate(100, 0)"><text y="-10">*YMax</text><rect x='.5' y='.5' width='29' height='59' fill='none' stroke='blue'/>
        <image xlink:href="http://www.deltalink.it/andreab/fabric/assets/smile.svg" preserveAspectRatio="xMaxYMax meet" 
             width="30" height="60"></image></g>
    </g>

    <g id="slice-group-1" transform="translate(100, 220)">
      <text x="0" y="-30">---------- slice ----------</text>
      <g><text y="-10">xMin*</text><rect x='.5' y='.5' width='29' height='59' fill='none' stroke='blue'/>
        <image xlink:href="http://www.deltalink.it/andreab/fabric/assets/smile.svg" preserveAspectRatio="xMinYMin slice" 
             width="30" height="60"></image></g>
      <g transform="translate(50,0)"><text y="-10">xMid*</text><rect x='.5' y='.5' width='29' height='59' fill='none' stroke='blue'/>
        <image xlink:href="http://www.deltalink.it/andreab/fabric/assets/smile.svg" preserveAspectRatio="xMidYMid slice" 
             width="30" height="60"></image></g>
      <g transform="translate(100,0)"><text y="-10">xMax*</text><rect x='.5' y='.5' width='29' height='59' fill='none' stroke='blue'/>
        <image xlink:href="http://www.deltalink.it/andreab/fabric/assets/smile.svg" preserveAspectRatio="xMaxYMax slice" 
             width="30" height="60"></image></g>
    </g>

    <g id="slice-group-2" transform="translate(250, 220)">
      <text x="0" y="-30">--------------- slice ---------------</text>
      <g><text y="-10">*YMin</text><rect x='.5' y='.5' width='49' height='29' fill='none' stroke='blue'/>
        <image xlink:href="http://www.deltalink.it/andreab/fabric/assets/smile.svg" preserveAspectRatio="xMinYMin slice" 
             width="50" height="30"></image></g>
      <g transform="translate(70,0)"><text y="-10">*YMid</text><rect x='.5' y='.5' width='49' height='29' fill='none' stroke='blue'/>
        <image xlink:href="http://www.deltalink.it/andreab/fabric/assets/smile.svg" preserveAspectRatio="xMidYMid slice" 
             width="50" height="30"></image></g>
      <g transform="translate(140,0)"><text y="-10">*YMax</text><rect x='.5' y='.5' width='49' height='29' fill='none' stroke='blue'/>
        <image xlink:href="http://www.deltalink.it/andreab/fabric/assets/smile.svg" preserveAspectRatio="xMaxYMax slice" 
             width="50" height="30"></image></g>
    </g>   
  </g>
</svg>
```
How do these properties behave for regular image objects in fabric? What do they do? How do they modify image rendering?