Add support for reading frame registers to Python API.

Add support for reading frame registers to Python API.

The ability to read registers is needed to use Frame Filter API to
display the frames created by JIT compilers.

gdb/ChangeLog:

2014-08-29  Sasha Smundak  <asmundak@google.com>

	* python/py-frame.c (frapy_read_register): New function.

gdb/doc/ChangeLog:

2014-08-26  Sasha Smundak  <asmundak@google.com>

	* python.texi (Frames in Python): Add read_register description.

gdb/testsuite/ChangeLog:

2014-08-26  Sasha Smundak  <asmundak@google.com>

	* gdb.python/py-frame.exp: Test Frame.read_register.