issue #990: provide a more flexible way of finding translated content

essentially all the "path + '.' + lang" code was replaced
with get_translation_candidate which honors the TRANSLATIONS_PATTERN
conf variable
use the old path.ext.lang pattern for translations by default

this is for backwards compatibility
but should be changed to the new default of something_lang.ext
in version 7
TRANSLATIONS_PATTERN: do not include dot in ext

just for convenience and readability
bugfix: forgot a paren
rebasing on master
rebasing on master
add TRANSLATIONS_PATTERN to default config
added more safe-checks for handing the config down
Merge branch 'translated_content_path' of github.com:smartass101/nikola into translated_content_path

Conflicts:
	docs/manual.txt
	nikola/utils.py