Use os.O_RDWR for exclusive mode in Python 2.7

[sqwishy]

Also doing the exclusive thing properly in openbin.

(#182)

I think the OS_RDWR behaviour is correct since if you specify x mode, you can't specify r or w. So it implies both. I'm pretty sure this is what Python 3 does so the fs package API should be consistent in both versions of Python here.

[coveralls]

Coverage remained the same at 99.754% when pulling cc96670 on sqwishy:rdwr-exclusive into cd1a973 on PyFilesystem:master.

[willmcgugan]

Thanks!