Start implementing a Pexpect spawn class based on Popen

Initial implementation copied from https://gist.github.com/blink1073/9a0ea82efc84cb9216d0
Some improvements
Restrict arguments that can be passed on to Popen
Enhanced popen_spawn and added tests

- Use just newline character for end of line
- Use shlex to split cmd args
- Improved non-blocking read to act more like ptyspawn
- Fix send method and add sendline
- Add kill method
- Add several tests
Add test file
Add a sendeof method and tests
Merge branch 'pr/146' of https://github.com/blink1073/pexpect
Allow import on win32
Fix test case for win32
Fix popen_spawn and test for windows.