Added glIs*

They're currently returning GLboolean, which is actually a u8. This is how the ll bindings expose them, so I kept it that way, but it'd be nice to just have them return Rust booleans. The current syntax looks something like:  
```rust
if gl::is_buffer(name) > 0
{ ... }
```
and it'd just flow better as:
```rust
if gl::is_buffer(name)
{ ... }
```

Let me know what you think.

Cheers,
Jeaye