Cstring escapes

This should fix #2. Please don't merge, yet, still working on this branch. Just wanted to publish, what I already got.

## ToDo

[x] cherry-pick mikejholly/node-po#3
[x] handle \n when writing
[] add more tests for mikejholly/node-po#3
[] implement reverse of c-string escapes for Item.toString (\n already done)