core: check for loop._ptr being NULL

raise ValueError if it is. Previously it crashed.

Closes #251. Thanks to Jan-Philip Gehrcke.
Replace self._threadpool.close() with self._threadpool.kill() in hub.py.

Closes #235.
Fix #263: potential UnboundLocalError: 'length'
rename sitesupport -> surfly
Simplify the db pool example (From PR #239)
Update pywsig.py, support GET can require Body
add a note on how to install from github

closes #258.
Merge pull request #264 from smallfish/master

Update pywsgi.py, support GET can require Body
fix PYTHONPATH in test___monkey_patching.py (#172)
README.rst: add 'running tests' section (#172)
changelog.rst: add dates to beta releases
changelog: copy entries for 0.13.7 and 0.13.8

from gevent013 branch