Added support for BigDecimal.

I think to have succesfully switched the project to use BigDecimal java object for sizes and prices.
I've successfully run the tests included in your project, after modifying them a bit ( obviously my tests and yours have the same semantics but differ in syntax).

I've also deleted the original QuickFixJ library and used this one (http://repo.marketcetera.org/maven/quickfixj/quickfixj-all/1.5.3-bd), so you'll have to manually add to your project, too.