Allow pointer types to have an attribute

Microchip's XC16 and friends, when used with dsPICs, require that all pointers
to memory which could possibly be in EDS space by adorned with __eds__, e.g.

    __eds__ int* p_int

Adding the macro UNITY_PTR_ATTRIBUTE allows Unity's pointers to be decorated
with whatever ridiculous attributes the compiler requires.
Merge pull request #51 from canton7/feature/ptr-attribute

Allow pointer types to have an attribute (file under "stupid things we do to keep poor compilers happy")