Simplify history feature

[jamowei]

Simplifying the history feature, which includes the following changes:

• history is synced with Store by default

// before
val store = object : RootStore<String>("") {
    val hist = history<String>().sync(this)
}
// now
val store = object : RootStore<String>("") {
    val hist = history() // synced = true
}

• renamed reset() method to clear()
• renamed add(entry) method to push(entry)
• removed last() method, cause with current: List<T> every entry is receivable
• changed default capacity to 0 (no restriction) instead of 10 entries

fixes #665