Add the functionnality to compare two XML files

XML files can be now compared by using the getDifferencesFromXml property type.
This type takes two arguments, the two files to compare, and returned a list of differences between them.

Difference list is formated by using the following XML representation:
<differences>
	<difference>/xpath/to/the/first/difference</difference>
	<difference>/xpath/to/the/second/difference</difference>
</differences>

Refer to issue #294
Merge branch 'master' of https://github.com/vertigo17/Cerberus