Ignore if .fmf dir exists.

[ssahani]

Closes #37

[psss]

Suggested code does not handle the conflict for existing directory:

> fmf init
Metadata tree '/tmp/exists/.fmf' successfully initialized.
> fmf init
Metadata tree '/tmp/exists/.fmf' successfully initialized.

And for existing file it does not work at all:

> rm -rf .fmf
> touch .fmf
> fmf init
Traceback (most recent call last):
  File "/usr/bin/fmf", line 35, in <module>
    fmf.cli.main()
  File "/usr/lib/python2.7/site-packages/fmf/cli.py", line 198, in main
    parser = Parser(arguments, path)
  File "/usr/lib/python2.7/site-packages/fmf/cli.py", line 73, in __init__
    getattr(self, "command_" + self.command)()
  File "/usr/lib/python2.7/site-packages/fmf/cli.py", line 146, in command_init
    if error.errno != errno.EEXIST:
NameError: global name 'errno' is not defined

Have you run it?

[psss]

I have fixed the issue in 364cf81.