3.0 compatibibility (Object::has_extension)

SiteTree::has_extension() is only available in 3.1,
split off 3.0 compat branch to avoid deprecation warnings.

See https://github.com/silverstripe/silverstripe-translatable/issues/98#issuecomment-14020318
BUG Make original field label translatable
NEW Better transformation of form fields (specifically CheckboxField)
Readded _config.php
FIX Retrieve list of translatable fields not database fields
More versatile travis script
Simplified travis builds (using external script)
FIX: getTranslations should not change the locale-filter-state

If the locale-filter was disabled before calling the method, it would be
enabled after calling getTranslations, this commit now only re-enabled
the locale-filter if it was enabled before calling the method...
Merge branch '1.0'

Conflicts:
	.travis.yml
	composer.json