Call hid_init() at import time, to avoid macOS crash

fixes trezor#142
fixes trezor#148

note trezor#148 (comment) :
> HIDAPI on macOS has an additional issue regarding thread-safety.
> I did encountered it in my project(s) but didn't have enough time to gather info/find a root cause.
> Looks like on macOS hid_init/hid_exit needs to be called in the same thread,
> which is an additional restriction compared to other platforms.

chid.pxd

@@ -23,6 +23,7 @@ cdef extern from "hidapi.h":
     hid_device* hid_open(unsigned short, unsigned short, const wchar_t*)
     hid_device* hid_open_path(char *path)
     void hid_close(hid_device *)
+    int hid_init()
     int hid_exit()
     int hid_write(hid_device* device, unsigned char *data, int length) nogil
     int hid_read(hid_device* device, unsigned char* data, int max_length) nogil

hid.pyx

@@ -8,6 +8,9 @@ from libc.stddef cimport wchar_t, size_t
 __version__ = "0.13.1"
 
 
+hid_init()
+
+
 cdef extern from "ctype.h":
     int wcslen(wchar_t*)