Made values optional for some assertContainerBuilderHas* assertions.

As discussed in #17, i made the check for concrete values for some assertContainerBuilderHas* assertions optional.

As you mentioned mattiasnoback, there has been a problem with optional paramters. For `assertContainerBuilderHasParameter` and `assertContainerBuilderHasServiceDefinitionWithArgument` i used `func_num_args` to determine, if a value for a parameter should be checked.
This was done because `null` would a possible value for a parameter, that might be want to be checked also.

If this is ok, the README needs to be updated.