Initial commit

Just a description and a simple Rust "Hello World" example.
First challenge WIP
Setup challenge1 with Cargo
Add base64 encoding and a few tests
Improve tests by using newly learned methods
Make encode_single_base64 use Option

Add some tests for that.
Use cmdline args for input in main.rs
Fancier error handling

Use Result<> for from_hex and add pattern matching or similar strategies
to the call sites.
Try to make from_hex more readable
A few more tests wrt .as_slice()
Initial challenge2
Basically working challenge2
More unit tests for challenge2
Try alternative approaches to some functions
Fix indentation
Initial version Challenge3
Make challenge3 work somewhat
Challenge 4 started
New module 'codec'

Moved code from challenge1 to new module and change challenge1 to use it.
Update challenge2 to use 'codec' module, too