Simplify v8::Value unwrapping.
Simplify v8::Isolate unwrapping.
Simplify v8::Context unwrapping.
Add v8::Context::Global() binding.
Add v8::Number bindings.
Remove Drop trait from v8::Isolate.

Auto-dropping the isolate makes it pretty much impossible to use the
isolate outside of the lexical scope that it was created in.  It also
doesn't play well with API functions that return a reference to the
current isolate, like v8::GetReturnValue::GetIsolate().