Removed the need to install lxml. 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.
Merge pull request #9 from djm/feature/remove-lxml-requirement

Removed the need to install lxml.