SnappyOutputStream should use internal buffer for writing chunk sizes

[xerial]

This code should be improved so that we can use the internal buffer instead of using BufferredOutputStream:
https://github.com/xerial/snappy-java/blob/develop/src/main/java/org/xerial/snappy/SnappyOutputStream.java#L275

[xerial]

@rxin
I just deployed snappy-java-1.1.1.2 to Maven central with this fix and some optimization in #83.

Memory usage is the same with the previous version, but it becomes faster for writing more than 1KB data by reducing the number of memory copies.