fix(database): remove impossible condition

an empty string will not equal undefined

style(build): remove extra space

How sad that the lint script had a linting error that wasn't linted!

fix(core): emit last value from observables

fix(core): replay last value from cache

This fixes a bug where initially the correct value is returned from a `list` or `object` call, but additional calls during the same session return `null`.

feat(core): support writing to Firebase offline

BREAKING CHANGE: new names for observables and primitives can only be accessed using `$value`

Before:

items: ListObservable<any[]>;
object: ObjectObservable<any>;

this.afo.database.list('/primative').subscribe(object => {
  console.log(object);
});

After:

items: AfoListObservable<any[]>;
object: AfoObjectObservable<any>;

this.afo.database.list('/primative').subscribe(object => {
  console.log(object.$value);
});

fix(core): remove angular-provide-once

feat(core): wrap AngularFire2 write methods

This only wraps the write methods already provided by AngularFire2, but does not enable offline support for writing to Firebase.

feat(build): default to tagging as @next

feat(core): support AOT

feat(index): remove forRoot method from @NgModule

BREAKING CHANGE: @NgModule no longer has a `.forRoot` method.

To migrate the your @NgModule code follow the example below:

Before:

imports: [
  AngularFireModule.initializeApp(firebaseConfig),
  AngularFireOfflineModule.forRoot(),
  BrowserModule
],

After:

imports: [
  AngularFireModule.initializeApp(firebaseConfig),
  AngularFireOfflineModule,
  BrowserModule
],

Allows the module consumer to not be concerned with the issue of instantiating multiple instances of the same service.