Remove leftover debug println!() statements from
decode_with_alphabet().

Moved into encode_with_alphabet()

One happy test case only for now.

Label functions with test status.

Fix documentation link.

Add pad char to the input alphabet.
Extract pad char from alphabet.
Remove pad char from alphabet.
Don't need pad char in encode lookup, but do need it in
the decode lookup.

Write tests for new pad char related helper functions.

Document encode_with_alphabet and decode_with_alphabet.
Add a list of links for navigation.
Add a notes section.

Move code from encode() to encode_with_alphabet()
Move code from decode() to decode_with_alphabet()
Refactor encode()/decode() to call the *_with_alphabet functions with
the "standard alphabet".
Add tests for the *_with_alphabet functions.
Leave encode()/decode() tests alone to ensure that backward
compatiblity isn't broken.

Names should be clearer.

- encode_with_alpha(...)
- decode_with_alpha(...)

Change name of input to encode() to something better...
Added better doc comments.

Add extra metadata to prepare for release.

Fixed one test vector.

Need more test cases though.

We're returning vectors, not arrays...  duh.