added possibility to use MkdirLockFile as an alternative

[psychok7]

I decided to create this PR because of #43 and in the end i realized that the problem was that i was using my Vagrant machine for development and it was not locking using the default FileLock from lockfile in my shared folder between the Guest and Host (maybe its related to hashicorp/vagrant#713 ).

in the aquire() function in the os.link() it was giving me OSError: [Errno 1] Operation not permitted

I then realized that lockfile also allows MkdirLockFile that provides a mkdir(2)() system call as the basic lock mechanism and it is even used on Windows systems.

Basically this PR allows the user to choose if he wants to use FileLock or MkdirLockFile where FileLock continues being the default.

[paltman]

@spookylukey we have a Slack setup for Pinax now with a channel dedicated to django-mailer. Want to join? http://slack.pinaxproject.com

[psychok7]

@spookylukey does this seem reasonable to you??

[spookylukey]

Actually, it seems overkill to have a setting for this.

If you find you need to change the setting, you've already wasted a lot of time debugging, probably. It should Just Work all the time, or have an explicit error if it can't work.

[ossanna16]

Any further discussion on this? Can this PR be closed?