Fixes for compression codecs

Motivation:

Fixed founded mistakes in compression codecs.

Modifications:

- Changed return type of `ZlibUtil.inflaterException()` from `CompressionException` to `DecompressionException`
- Updated throws tags in javadoc of `JZlibDecoder` to throw `DecompressionException` instead of `CompressionException`
- Fixed `JdkZlibDecoder` to throw `DecompressionException` instead of `CompressionException`
- Removed unnecessary empty lines in `JdkZlibEncoder` and `JZlibEncoder`
- Removed public modifier from `Snappy` class
- Added `MAX_UNCOMPRESSED_DATA_SIZE` constant in `SnappyFramedDecoder`
- Used `in.readableBytes()` instead of `in.writerIndex() - in.readerIndex()` in `SnappyFramedDecoder`
- Added private modifier for `enum ChunkType` in `SnappyFramedDecoder`
- Fixed potential bug (sum overflow) in `Bzip2HuffmanAllocator.first(...)`. For more info, see http://googleresearch.blogspot.ru/2006/06/extra-extra-read-all-about-it-nearly.html

Result:

Fixed sum overflow in `Bzip2HuffmanAllocator`, improved exceptions in `ZlibDecoder` implementations, hid `Snappy` class