Oembed warning if GD function is missing

Silencing errors from a function is terrible so I've stopped that.

I've also added detection for if `imagecreatefromstring` is a declared function and a useful error is thrown if not
BUG HTMLText whitelist considers text nodes
Minor improvement to #2853.
If a list of whitelisted elements are specified, text nodes no longer evade the whitelist
Fix 'Uncaught ReferenceError: jQuery is not defined' if jQuery is not included in template.
Merge pull request #3070 from dhensby/patch-3

Oembed warning if GD function is missing
FIX folder Title not being exactly the same as Name field if setName modified the value
Change PHP 5.6 to an allowed failure
Merge pull request #3086 from hafriedlander/fix/CWPBUG-155

FIX Folder Title not being exactly the same as Name field
BUG Allow PHPUnit installation with composer / Fix travis
Merge pull request #3088 from tractorcow/pulls/3.1-phpunit

BUG Allow PHPUnit installation with composer / Fix travis
Merge pull request #3079 from timsnadden/3.1

Fix 'Uncaught ReferenceError: jQuery is not defined'
Don't add the class manifest with tests as an exclusive manifest
Merge pull request #3096 from simonwelsh/nonexclusive-test

Don't add the class manifest with tests as an exclusive manifest
Merge pull request #3021 from tractorcow/pulls/3.1-htmltext-whitelist

BUG HTMLText whitelist considers text nodes
Path resolution cleanup
See security@silverstripe.com
[ref: CWPBUG-158]