Misc. grammar fixes for the memory chapter.

remove_message does not store any pointer to x0

On OTP 20 erlang compiler generates following code:
```
{get_tuple_element,{x,0},1,{x,0}}.
remove_message.
```

x0 is clearly still used and it shouldn't be overwritten by
remove_message, also in msg_instrs.tab only loop_rec has
`r(0) = ERL_MESSAGE_TERM(msgp);`.

another spelling fix

Generate opcodes file.

Build contributors.txt once again

Asciidoctor (happi#66)

* Use asciidoctor to generate HTML version of the book
* Fix a few diagrams
* Style images
* Additions to .gitignore file
* Minor fixes

Moving chapter files to chapters dir. (happi#33)

* Moving chapter files to chapters dir.

* Adding .gitignore to make sure we have an ebin.

* Add .gitignore to make keep xml dir.

* Fix shell language.

* Remove revision history and add git contributers.

* Fix build.

* Fixing contributors generator.