Extraction

This PR got a bit messy compared to the other one. This is because it gets diff'ed against master and not the other PR.

The only additions from the last one is the two files:
- gettexter_extract.erl
- gettexter_po_writer.erl

gettexter_extract defines a function extract/0 which is meant to be run from the CLI. This should provide a simple but powerful default to extract everything from the src/ directory. Options can also be provided via the CLI. So far this is only the ground work and this can be improved a whole lot. Hopefully I've made it easy to extend. I'd like to support more options both when extracting and when building the PO-files. There will be an additional PR that should add better documentation to repo.
For strings without a domain (AKA default domain) we get `undefined` as the domain here, right? Normally, the file for those strings should be `messages.pot`.