Marke Encoder as Send + Sync to enable multithreaded use (#4)

To quote LAME 3.99.5 HACKING:
THREADSAFE:

Lame should now be thread safe and re-entrant. 
The only problem seems to be some OS's allocate small
stacks (< 128K) to threads launched by applictions, and this
is not enough for LAME.  Fix is to increase the stack space,
or move some of our automatic variables onto the heap with
by using bug-prove malloc()'s and free().

src/lib.rs

@@ -622,6 +622,11 @@ impl Drop for Encoder {
     }
 }
 
+/// According to LAME 3.99.5 HACKING, it is thread-safe.
+unsafe impl Send for Encoder {}
+/// According to LAME 3.99.5 HACKING, it is thread-safe.
+unsafe impl Sync for Encoder {}
+
 ///Creates default encoder with 192kbps bitrate and best possible quality.
 pub fn encoder() -> Result<Encoder, BuildError> {
     match Builder::new() {