Split Core.php into Constants.php and Core.php and adjust main.php startup

The recent flush filter fix had a problem that you couldnt set a custom
BASE_PATH in _ss_environment because that file didnt get included until
after checking the confirmation token. This patch pulls the part of Core.php
that defines BASE_PATH into a seperate file that can be included earlier
in the startup sequence so that ParameterConfirmationToken can access it.

Core.php includes Constants.php with a require_once call, so for startup
scripts that dont pull in Constants.php themselves (like cli-script.php)
no change is needed.
FIX Only suppress fatal errors
Merge pull request #2255 from hafriedlander/fix/flush_30

Split Core.php into Constants.php and Core.php and adjust main.php startup
Merge pull request #2256 from tractorcow/3.1-setratiosize-dbz-fix

BUG Fixed divide by zero with SetRatioSize on missing image file
Fixed CountryDropDownField showing East Germany
FIX Nice errors and allows flush on module removal
Merge pull request #2260 from ARNHOE/3.1-fixedeastgermany

Fixed CountryDropDownField showing East Germany
Merge pull request #2257 from hafriedlander/fix/flush_30

FIX Nice errors and allows flush on module removal
Merge branch 'origin/3.0' into 3.1
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 misleading documentation about Children.max

Currently, the documentation implies that doing a `Children.max(LastEdited)` will work, which isn't the case.
This change uses `AllChildren.max(LastEdited)` instead, which while slightly more inefficient, will actually work consistently.
Merge pull request #2279 from ss23/patch-7

Fix misleading documentation about Children.max in partial caching
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
Merge pull request #2284 from wilr/closure_this_reference_fix

FIX: pass controller as reference inside closure.