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.
Added support for a Decimal field which coerces raw input values to Python's Decimal type (available since 2.4). Added supporting tests.
Removed accidental extra linebreak.
Merge branch 'feature/add-decimal-field'
Merge branch 'feature/remove-lxml-requirement'

Conflicts:
	adaptor/fields.py - lxml import removed.