Allow for explicit default loop creation via `get_hub(default=True)` after default loop destruction. Include unit test.

also run 'apt-get upgrade' after 'apt-get update'.
move apt-related commands to travis_install_deps.sh

it could be that self.greenlet was reset to None by the time handle_error is called

see https://github.com/SiteSupport/gevent/issues/181 for a discussion

it's not always correct. for example, Popen immediatelly execs another process,
so no Python objects are actually leaked.

this is issue https://github.com/SiteSupport/gevent/issues/116

the mimetools Message class isn't of much help here, but replacing it
probably will do more harm than good. so, we parse multiline headers
on our own.

The
,----
| if 'bytearray' in __builtins__.__dict__:
`----
fails when running under py.test, so I replaced that too.

make sure we deliver exception to MAIN greenlet even if loop.run_callback fails

- update test__socket_close.py

previously it was added to the queue of callbacks.
Update test__systemerror.py

it seems to support them since 0.3.1

- added rawlink() method to Popen
- added linkproxy helper class to gevent/hub.py
- modified examples/processes.py to use gevent.wait()

- rename os.threadpool_read/write -> os.tp_read/write
- replace os.posix_read/write with os.nb_read/write must be only used on non-blocking file descriptors
- add os.make_nonblocking function to make fd non-blocking
- ignore test_thread.TestForkInThread.test_forkinthread

now run_callback is based on a list and a single prepare watcher

- remove loop.callback
- update sleep(0) to use run_callback (and to truly switch to other greenlets)
- updated related tests