Moved bit-level read operations from Bzip2Decoder to the new Bzip2BitRea...

...der

Motivation:

Collect all bit-level read operations in one class is better. And now it's easy to use not only in Bzip2Decoder. For example, in Bzip2HuffmanStageDecoder.

Modifications:

Created a new class - Bzip2BitReader which provides bit-level reads.
Removed bit-level read operations from Bzip2Decoder.
Improved javadoc.

Result:

Bzip2BitReader allows the reading of single bit booleans, bit strings of arbitrary length (up to 24 bits), and bit aligned 32-bit integers.

@trustin @normanmaurer wdyt?
add to exception why it is illegal.