Add initial header tables in header_common.js
Implement HeaderEncoder.encodeHeaderBlock()

Also implement EncodingContext.addInitialHeader().
Separate Encoder from EncoderContext
Move Encoder to header_encoder.js
Add stub HeaderDecoder
Move common code into encoding_context.js
Make encoder a local variable in HeaderEncoder
Flesh out HeaderDecoder.decodeHeaderSet()
Fix bug with a misuse of JavaScript's division operator

Also add some comments.
Implement encoding of header names existing in the header table
Handle indexed entries
Use incremental indexing
Use substitution indexing

Fix a bug with the opcodes.
Use new code in test file
Display contexts
Fix bug with max size handling