Rename default environment variables (PATH_INFO_ORIG, LANGUAGE_TAG)

* Rename default environment variables (PATH_INFO_ORIG, LANGUAGE_TAG) into more safer ones (extracturilanguage.path_info, extracturilanguage.language)
* lexical constants for environment variable names (Type.pm)
Add ExtractUriLanguageList

Fixes #3
Only detect and extract the language tags defined with this list. The default is "undef". When "undef" ExtractUriLanguage will try to guess the language tag.
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