Build problem on OSX

Function declaration in `src/period.h`

```
int get_yq(int64_t ordinal, int freq, int *quarter, int *year);
```

doesn't match definition in `src/period.c`

```
int get_yq(npy_int64 ordinal, int freq, int *quarter, int *year) {
```

Changing `int64_t` to `npy_int64` in `period.h` fixes the build and all tests pass on OSX.
