Add usage of xpath to localise components in web pages


## Context
In many case, J2EE/Web frameworks generate their own "id" attribute for components,
so it is impossible to write tests using only identifiers

## Description
Xpath attributes have been added to the following web steps:
* AssertCheckBox
* AssertPresent
* AssertText
* ClickButton
* ClickLink
* ClickCheckBox

For more information on Xpath expressions: see [W3School Xpath tutorial] (http://www.w3schools.com/xpath/).

## Example


Html sample:
```html
<div id="emptyBox" class='boxClass'>
    <div class='boxClass'>
        <h5 id="handle">originBox</h5> 
        <p>CONFLUENCE</p>
    </div>


    <div class="boxClass">
        <h5 id="handle_two">originBox</h5> 
        <p>MINT</p>
    </div>
</div>

<p>
    <a href="dragAndDrop.html" id="dragAndDropPageLink">Drag and drop page</a>
</p>
```

Test release xml sample:
```xml
<!-- click on the "Drag and drop page" link -->
<clickLink xpath="//a"/> 
<!-- same thing with id attribute -->
<clickLink xpath="//a[@id='dragAndDropPageLink']"/> 

<!-- localisation with css class name -->
<assertText containerXpath="(//div[contains(@class,'boxClass')])[1]">CONFLUENCE</assertText>
<assertText containerXpath="(//div[contains(@class,'boxClass')])[2]">MINT</assertText>
```