Support the '&' character for types implementing multiple interfaces

[nicholaslythall]

Added Token.Kind.amp Updated lexer.readToken to handle ampersands Updated parseImplementsInterfaces to support interface names separated by &, e.g:

type Swallow implements Bird & Animal {}

[paulofaria]

Sorry for the delay! Looks like the branch is out-of-date with master. Can you please update from master?