Allow mapping of generic methods in expressions irrespective of the parameter

[BlaiseD]

Partial fix for issue #12

Now it's possible to map the following:

Expression<Func<OptionS>> exp = () => Activator.CreateInstance<OptionS>();
Expression<Func<OptionT>> expmapped = mapper.Map<Expression<Func<OptionS>>, Expression<Func<OptionT>>>(exp);

[BlaiseD]

@jbogard - Does AutoMapper.Extensions.ExpressionMapping have a MyGet?

[BlaiseD]

@jbogard - never mind - I see it