Change leader in vimrc to ','

The default leader was not very convenient. I saw that
other people who use leader remap it to ',' and it
turned out to be much better. It renders one of the
default mapping unusable, but I use it very rearly if
at all, so it should be ok.
Add check for vim version before setting colorcolumn

* The feature 'colorcolumn' is supported after vim 7.3. In order not to
  have problems with this with earlier version of vim, I added a check
  for the current version that is being used.
Add the plugin 'turbux'

* This plugin makes it easier to start tests in ruby, especially with
  rails and for that reason I added it.
* I adjusted some of its options and those of vimux so that it is more
  convenient for my workflow.
* I removed an old mapping from back in the days when I did not use
  'nerdcommenter'
Improve gitconfig

* Make merge ff-only by default.
* Make vimdiff the default merge tool.
* Disable prompt before mertool is used.
* Set ui color to auto as otherwise on some computers there is no color.