Add support for the new onerror attribute in CFFI 1.2.0

Add support for the new onerror attribute in CFFI 1.2.0/PyPy 2.7.0. This fixes our signal handling issues.

Support more objects that implement the buffer protocal by asking for the len() of their memoryview, not the len() of themselves.

Also potentially reduces the number of calls to memoryview so may be slightly more efficient.

Fixes #466.

lint; those tests were fixed with the previous commit but travis is having issues getting builds off the ground

If the environment had not been monkeypatched when gevent's ssl module
was imported, they could get the stdlib socket, meaning that reads and
writes could block greenlet switching for an arbitrary amount of time.
This was especially visible on SSL servers.

Fixes #597.

It preserves the tracebacks and obsoletes the ``apply_e`` function.

Add test cases for this behaviour.

Fixes #556.

This ensures ThreadPool has the same API, including accepting multiple iterables in imap.

Also, capture the _raise_exception method of a failed greenlet and use it to raise the original traceback from those methods.

Avoid potential reference cycles by pickling the tracebacks a Greenlet stores

Patch subprocess by default. Fixes #466.