Make sure run_callback() (and thus sleep(0)) does not starve CPU.
After some number of callbacks (1000), let the event loop run.

Fixed bug in --module argument for gevent.monkey

Logical error making it impossible to specify separate modules to
enable. Credits to leiqin for the fix[1].

[1] http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=694750

Also now the --module argument invocation follow the documentation and
will disable all other modules not explicitly specified to be enabled.
It's now possible to specify additional options, eg. --[no-]aggressive.

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