Try both ~/.netrc and ~/_netrc files #186

Windows uses "_netrc" rather than ".netrc".
Our implementation should simply try both.

We might want something like this...

if 'NETRC' in os.environ:
    NETRC_FILES = (os.environ['NETRC'],)
else:
    try:
        NETRC_FILES = (os.path.expanduser('~/.netrc'), os.path.expanduser('~/_netrc'))
    except KeyError:
        # May be raised if no user home directory is set.
        NETRC_FILES = ()

As part of this change we probably also want to make sure that any netrc file permission error or parse error simply results in netrc permissions not being used, rather than hard erroring.

Related to #177