Pull feature

Using work of drench and yellowtree I propose a new workflow for dealing with remote changes.

Like Git, git-ftp push aborts if remote changes were detected (timestamp checking only). It suggests to pull changes first. The adaption of git-pull checks out the last deployed version, downloads the changes using 'lftp mirror' and creates a new commit in an unnamed branch. The SHA1 is uploaded so that everything is in sync again. Last, it goes to your previous branch and performs a git-merge. The option --no-commit allows to easily review all remote changes and to define an own commit message.

In addition, the new commands bootstrap and download are available (drench's bootstrap and fetch).

This should solve the issue #29, obsolete the pull-request #52 and may solve issue #13.

The new features are unit-tested. Feedback is welcome. :-)