don't require both email and username. closes #32

This change breaks backwards compatibility.

`createClient` no longer takes an `email` argument. Instead, the
`username` argument is used to authenticate and determine the
case correct username.

There is a special case if you leave out the `password` argument.
In this case, `username` is used to connect directly to the server,
and you may get kicked if the server is in online mode.
Release 0.7.0
batch is a devDependency only now
README still had MC_EMAIL instead of MC_USERNAME

The tests use the MC_USERNAME environment.
Merge pull request #33 from roblabla/patch-1

README still had MC_EMAIL instead of MC_USERNAME
Removed socket delays to reduce latency
Merge pull request #34 from mappum/master

Removed socket delays to reduce latency
Release 0.7.1
more robust 0x28 test
fix objectData writer
Release 0.7.2
Revert "Removed socket delays to reduce latency"

This reverts commit 6442e8d6b06b92d4c8d253bee7c91f91f6456d2c.

Reverting until we can figure out why this is causing test failures.
client emits more predictable 'end' event
Release 0.7.3
server: online mode: don't log in client until username verification
Release 0.7.4
ability to exempt usernames from online mode or offline mode
Release 0.7.5
fix case hole in onlineModeExceptions
Release 0.7.6