docs: replace unicode padlock glyph with :lock:

My editor shows a big white space where the padlock icons were supposed
to go. The word "lock" sandwiched by colons is readable in plain ASCII,
and github markdown renders it as the same padlock icon.
Merge pull request #95 from warner/lock-icon

docs: replace unicode padlock glyph with :lock:
added error.js for custom errors
many changes to client module, started integration tests