Added statusline settings. Fixed indentation

I want to use a custom statusline for both Gundo splits so i added two
variables for it setting it. If the variables are empty Gundo works like
normal but if there is something in them the text get used. Example:
  let g:gundo_preview_statusline = "Gundo Preview"
  let g:gundo_tree_statusline = "Gundo Tree"

The indentation was off in a few places so i fixed it. The real tabs was
removed and replaced with spaces like the rest of the source file.
Add documentation for the statusline options.
Added tag v2.3.0 for changeset bf31800e9784
Add auto preview global option
Add auto preview global option
Add r mapping to preview current status
Add r mapping to preview current status
Add gauge.
added public show/hide methods and matched toggle open/close method
Update lessjs.
Merge.
Update changelog.
Update site.
Update help text a bit.
Added tag v2.4.0 for changeset 25b74fe299c3
Whitespace.