No more http dependency, some meta functions

• Removes fetchTranslations and the dependency on elm/http. Since the elm compiler does not support having multiple version of the same library it is better not to depend on elm/http so that users can choose more freely whether to use elm/http version 1 or 2. Also, I assume that fetchTranslations was not highly used and can easily be built by users.
• Add keys and hasKey function. This should not be needed for regular applications but exposing them allows makers of translation editors to query for missing keys or check if keys exists.
• Add a proper example, in which the default case is passing the translations via flags. The case where Elm makes an Http request for the translations file should be added soon.