FIX: GridField button styling

Remove all top/bottom margins from buttons and apply to GridFieldButtonRow component. Ensure that all buttons are added to a suitable GridFieldButtonRow in ModelAdmin, SecurityAdmin and Group.
FormFields now allow setting of extra CSSClasses en masse

Each CSS class passed in to `addExtraClass` or `removeExtraClass` will be set as their own key in the `extraClasses` array

Also make `Form` consistent with `FormField`
Adding tests to `FormField` and `Form` for extra classes

Added tests that were missing from `Form` and ones for my new logic
FIX Empty Datefield with defined min or max has non-object error thrown

When submitting a Datefield with no value but with a min / max config date, the validate() function attempts to access a function on $this->valueObj (which is a non-object)
Updated field class to use doubleborder mixin
BUG Fieldgroup styling
fixed undefined offset #2276
Minor bug fix to show link description field 

When adding a link, the description field was present, but hidden. Adjusted javascript to show the description field
FIX: pass controller as reference inside closure.

Non referenced causes segfaults on PHP <= 5.3.8 environments when opening subtrees in the CMS. Likely the cause of silverstripe-cms/issues/803.
Changelog note on form method limitations

See 14c59be85e9ec53cb2f0b3e9d0d8ce7569d913c6.
Raised by Fara Rustein of Deloitte Argentina (CVE-2013-2653).
FIX Flush on memory exhaustion and headers sent
FIX Include flushtoken when install redirects to successfullyinstalled
API Disable discontinued Google Spellcheck in TinyMCE

Replaced by browser-based spellchecking if available (Chrome, Firefox),
with instructions on how to use PSpell as an alternative.
Merge pull request #2284 from wilr/closure_this_reference_fix

FIX: pass controller as reference inside closure.
Merge pull request #2278 from jonshutt/patch-1

Minor bug fix to show link description field
Merge pull request #2277 from joernroeder/3.1

fixed undefined offset #2276
Docs: Consistent arg quoting in <% require %> (#1614)
Merge pull request #2271 from ARNHOE/3.1-twotoneborders

Updated field class to use doubleborder mixin
FIX Token redirect where in IIS a / needs adding between host & url
FIX Constants magic_quotes handling needs function from Core