makefile: use new `jekyll serve` command
link to error codes from api call sections
fixup
`code` tags on successive lines shouldn't overlap
change title to Bitme REST API
update intro section wording
reorganize authentication api calls

* move Errors section before Authentication section
* move login, logout, join to 'User Management' section
* move GET /verify-credentials to Authentication section
Merge pull request #2 from bitme/decoupled1-sean

Decoupled1 - Sean's edits