Adds isPrimitiveType flag to CodegenParameter.

[chakrit]

This is required in some languages like Obj-C where there is no automatic value boxing and
handling of primitive types require extra work compared to non-primitive types.

Case in point: BOOL type. To assign BOOL into a dictionary, one needs to box it into an
NSValue * instance, so to build a dictionary for sending form data, for example, you
cannot do this in the template:

dict["{{paramName}}"] = {{paramName}};

Because if the parameter ends up being of type BOOL, an error about boxing values will be
generated:

BOOL boolValue = NO;
dict["boolValue"] = boolValue;
                    ^---------------- Cannot do the assignment here.

The fix is to wrap it in @() like so:

BOOL boolValue = NO;
dict["boolValue"] = @(boolValue);

So a flag is needed in CodegenParameter so we can selectively emit the right boxing or
non-boxing assignment in the templates where needed.

[chakrit]

We have a copy of our own templates (modified from the work in #754) at the moment. I will try to add fixes to the master branch template files where applicable if I have the time.

[wing328]

@chakrit please kindly submit the PR for the branch develop_2.0 instead of master