fix(classDetails): add support for do in the constructor

Closes #8
refactor(app): don't assign directly to the constructor

BREAKING CHANGE: use return value from constructor

Before:
class App extends App
  @constructor = []

After:
class App extends App
  constructor: ->
    return []
docs(README): update App docs
refactor(constant): don't assign directly to the constructor

BREAKING CHANGE: use return value from constructor

Before:
class MyConst extends Constant
  @constructor = []

After:
class MyConst extends Constant
  constructor: ->
    return []
refactor(value): don't assign directly to the constructor

BREAKING CHANGE: use return value from constructor

Before:
class MyValue extends Value
  @constructor = 'Hello'

After:
class MyValue extends Value
  constructor: ->
    return 'Hello'
docs(README): update How templates