Refs #38, fix pywsgi headers issue in PY3

Ignore sys.exc_clear() for PY3.

Fix indexing exceptions (ex[0]) by using args (ex.args[0]), refs #38

Refs #38, fixed renamed symbols in PY3

Fix links for examples (docs gevent servers)

Fixed exec, refs #38.

Replaced execfile() with exec(open().read()) in gevent/monkey.py.

upgrade libev to 4.15

This should fix #361.

not needed since 2.5 is not supported anymore.

- do not swallow syntax errors
- make pyflakes.py py3-compatible #38
- run using specific version of python (previously pyflakes from the default python
  was used, which caused it to ignore py3-specific and py2.6-specific syntax errors

Previously pep8 and pyflakes were installed using the default system Python.
This changes 'make travis' to use the current Python version for those scripts as well.
This affects pep8, at least: on Python3 it complains about space in `print (...)`.

Handle renaming of "thread" to "_thread" in PY3

Use six.xrange in tests.

the subprocess.py warning was actually legit and was fixed in 47f1c3d5

Py3-related random fixes

since gevent.os is imported in __init__.py this makes gevent.socket imported in __init__.py which was never the intention.