Add Python InferiorThread.inferior attribute

Add Python InferiorThread.inferior attribute

So a script can easily get at a thread's inferior and its number.

gdb/ChangeLog:
2016-01-13  Pedro Alves  <palves@redhat.com>

	* NEWS: Mention InferiorThread.inferior.
	* python/py-infthread.c (thpy_get_inferior): New.
	(thread_object_getset): Register "inferior".

gdb/testsuite/ChangeLog:
2016-01-13  Pedro Alves  <palves@redhat.com>

	* gdb.python/py-infthread.exp: Test InferiorThread.inferior.

gdb/doc/ChangeLog:
2016-01-13  Pedro Alves  <palves@redhat.com>

	* python.texi (Threads In Python): Document
	InferiorThread.inferior.