Added support for a Decimal field which coerces raw input values to Python's Decimal type (available since 2.4). Added supporting tests.
 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.