Proposal: more descriptive keys for Timber i18n (WIP)

Opening discussion for some tweaks to keys in i18n-enabled themes. The idea here is that keys will be used as labels or tooltips to provide context for a given translation.

For instance, `submit_button_text` is more descriptive than `submit`

This is how descriptive keys currently look in the editor, as labels:

![screen shot 2014-09-15 at 5 38 51 pm](https://cloud.githubusercontent.com/assets/785/4279379/b7c7bd84-3d20-11e4-8590-c14ec37657e1.png)

Relevant changes are (mostly) in this diff: https://github.com/Shopify/Timber/commit/1a74490a6884ba6aab1838b155f2321ef3c17903?diff=split

cc @cshold @fredryk @jonasll