Serde support for point types

Please add optional support for serde's Serialize/Deserialize traits.
Like this:
```toml
serde = { version = "1.0", features = ["derive"], optional = true }
```
```rust
#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
pub struct Point { // postgis::ewkb::Point
```
Thanks!