[WIP] Added the ability to skip examples

This is the first step of the topic related in the issue #119.
Second step would be to introduce guards (through example annotations) that will check some conditions (php version, package version, extension avaibility, etc...) and throw SkippingException for the user.

## Dot formatter
![capture du 2014-02-27 12 20 47](https://f.cloud.github.com/assets/668604/2281697/4af8af1e-9fa1-11e3-88ee-8b9c7cfcf432.png)

With the verbose option activated:
![capture du 2014-02-27 12 21 12](https://f.cloud.github.com/assets/668604/2281705/74037b00-9fa1-11e3-87c1-184abccc46b4.png)

## Progress formatter
![capture du 2014-02-27 12 32 01](https://f.cloud.github.com/assets/668604/2281781/e6961618-9fa2-11e3-9cb8-39f05eb25e0e.png)

With the verbose option activated:
![capture du 2014-02-27 12 32 18](https://f.cloud.github.com/assets/668604/2281783/ee415b48-9fa2-11e3-83a4-f0c48d3015e6.png)

## Pretty formatter
![capture du 2014-02-27 12 37 57](https://f.cloud.github.com/assets/668604/2281838/b2fe7376-9fa3-11e3-9d96-a5ac4e855b9e.png)

With the verbose option activated:
![capture du 2014-02-27 12 38 20](https://f.cloud.github.com/assets/668604/2281841/babdd354-9fa3-11e3-88f5-4b5cc8b8f785.png)

## JUnit formatter
``` xml
<?xml version="1.0" encoding="UTF-8" ?>
<testsuites time="0.067584991455078" tests="8" failures="0" errors="0">
    <testsuite name="PhpSpec\CodeGenerator\Generator\ClassGenerator" time="0.067502021789551" tests="8" failures="0" errors="0" skipped="1">
        <testcase name="it is a generator" time="0.014213800430298" classname="spec\PhpSpec\CodeGenerator\Generator\ClassGeneratorSpec" status="passed" />
        <testcase name="it supports class generations" time="0.0051472187042236" classname="spec\PhpSpec\CodeGenerator\Generator\ClassGeneratorSpec" status="skipped">
            <skipped><![CDATA[ skipped: This is incompatible for some reason ]]></skipped>
        </testcase>
        <testcase name="it does not support anything else" time="0.0057218074798584" classname="spec\PhpSpec\CodeGenerator\Generator\ClassGeneratorSpec" status="passed" />
        <testcase name="its priority is 0" time="0.0049088001251221" classname="spec\PhpSpec\CodeGenerator\Generator\ClassGeneratorSpec" status="passed" />
        <testcase name="it generates class from resource and puts it into appropriate folder" time="0.012134790420532" classname="spec\PhpSpec\CodeGenerator\Generator\ClassGeneratorSpec" status="passed" />
        <testcase name="it uses template provided by templating system if there is one" time="0.0092339515686035" classname="spec\PhpSpec\CodeGenerator\Generator\ClassGeneratorSpec" status="passed" />
        <testcase name="it creates folder for class if needed" time="0.0084249973297119" classname="spec\PhpSpec\CodeGenerator\Generator\ClassGeneratorSpec" status="passed" />
        <testcase name="it asks confirmation if class already exists" time="0.0066289901733398" classname="spec\PhpSpec\CodeGenerator\Generator\ClassGeneratorSpec" status="passed" />
    </testsuite>
</testsuites>
```

## HTML formatter
![capture du 2014-02-27 14 37 21](https://f.cloud.github.com/assets/668604/2282851/5191ae26-9fb4-11e3-8f36-c2597f8d39eb.png)