A couple of small fixes removing unneeded variables

It's unnecessary and breaks parallel build. Closes #231, #193.

Previously it was called 'receiver'. Also link_exception() had
a default value for it which was None, but it did not work,
since linking to current greenlet feature was removed some time ago.

Section on spawn_link was removed from doc since these functions have
been gone for some time as well.

Closes #244.

the old .deb packages were 32bit and travis is now 64bit

also update known_failures.txt w.r.t python path

This problems seem to have nothing to do with gevent:
SSLError: [Errno 1] _ssl.c:504: error:14094415:SSL routines:SSL3_READ_BYTES:sslv3 alert certificate expired
SSLError: [Errno 1] _ssl.c:504: error:14090086:SSL routines:SSL3_GET_SERVER_CERTIFICATE:certificate verify failed

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.