contextlib: Add contextlib from cpython 3.4.2.
contextlib: Make compatible with micropython
select: Clarify comment.
os: check_error(): Return true for EINTR, to easily restart system calls.
select: Handle EINTR.
signal: Add basic signal implementation (only IGN & DFL are supported).
signal: Add test for SIG_IGN functionality.
signal: Implement support for Python handlers.
signal: Add example of signal handler in Python.
time: Initial cut at implementing strftime() (via ffi).