amberc.js: change some defaults into direct members

Those properties should stay constant if the compiler
gets used multiple times.
The defaults are an empty skeleton which gets changed
throughout each run of amberc.
amberc.js: export AmberC constructor as exports.Compiler
amberc.js: amber_dir and closure_jar are constructor parameters now
amberc.js: rename map() to async_map()
amberc.js: also export Combo object and async_map()
amberc.js: update doc on how to use amberc