* added Solidity

* added Solidity

* added Solidity

* bitcoin book added

* fixed ordering on bitcoin book entry

* added authors for bitcoin and solidity books

* added Bitcoin and cryptocurrency Princeton book

* elm interactive playground added (ellie)

* free elm books added

* elm podcast added

* corrected spacing

* reordered elm books into existing category

* reordered format and author

* removed trailing /s

* fixed alphabetical ordering