Added a section that explains how to get the `transfer` alias on Windows.

Implement support for custom s3 endpoints

Switch to alpine base image

This drastically decreases the overall image size. Quick comparison:

```
# Image fetched from Docker Hub
dutchcoders/transfer.sh   latest              a875ed5dfa56        5 months ago        704MB
# Image just built
dutchcoders/transfer.sh   latest              a220c49297eb        4 minutes ago       269MB
```

- support for max downloads
- support for expiring downloads

* use dep for vendoring
* lets encrypt
* moved web to transfer.sh-web repo
* single command install
* added first tests

trivial spelling fixes

* reported by Tim / polym

SYMBOLS was missing the characters T and W