Type-checking when getting values

[jasongin]

There were a few questions in the PR about whether we need to check the type of a value before getting a casted value. For example, in napi_get_value_int32 we check value->IsNumber(), then call value.As<v8::Int32>()->Value(). What if the value is a non-integer number? Does the As<> cast work, or crash, or what?

Basically we need to add a bunch of test cases for these kinds of conversions, and add necessary type checks to avoid crashes. In my opinion, the APIs should not crash when provided a napi_value that is valid but the wrong type.

[jasongin]

Fixed by #194