Removed the need to install lxml.

Hi,

Thanks for your time on this project!

As per the commit message:

>This package can be used as a CSV importer without the requirement of lxml and therefore I believe lxml should be an optional extra for when working explicitly with XML:
- Removed from requirements.txt
- Moved module level lxml imports to the method level; ideally the XML fields should be seperated off into another module so that this is not required.
- Moved lxml requirement in setup.py from install_requires to extras_require.

Thanks,
Darian