Remove LXML for 50% faster builds!

This was brought up in https://github.com/edx/diff-cover/issues/92 
Trying to get buy in at my company the slow build was a problem... so I am trying to address it

Remove dependency on lxml as it leads to slow install and if a project does not already have lxml then diff-cover significantly slows down the build
cElementTree is also faster than lxml for a lot of things but probably not noticeable so for our use case

This PR is rather risky. The tests pass but I am not an expert on the coverage xml file. 

For the record, *this would have been a drop in replacement if it were not for python2.6* ::pout::

Before
![screen shot 2014-07-16 at 12 02 59 am](https://cloud.githubusercontent.com/assets/1521093/3594479/dfc25c84-0c9e-11e4-946f-f18fa689281a.png)

After
![screen shot 2014-07-16 at 12 07 51 am](https://cloud.githubusercontent.com/assets/1521093/3594480/e431a054-0c9e-11e4-95d5-5e0ad8fb5166.png)
Sure
```
[class_tree for class_tree in xml_document.findall(xpath) or []]
```
if ```xml_document.findall(xpath)``` returns None this will iterate over and empty list instead of exploding 

for example
```
>>> [x for x in None]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'NoneType' object is not iterable

>>> [x for x in None or []]
[]
```

what is happening is the or evaluates first returning the list to iterate over.