XEP-0004: Data Forms support
Remove unexported identifiers from features

Fixes #13
compress: Add support for LZW stream compression

Also tweak some comments to make later diffs smaller.

Fixes #15
form: rename "varName" to "id"
form: Rename Option and FieldOption types

Renamed to "Field" and "Option" respectively.
ibr2: Always send a response
ibr2: Implement OOB challenges
ibr2: Add initial negotiation implementation
ibr2: Add API stability disclaimer to docs
ibr2: Fix broken string formatting in tests
Add an experimental IBR2 API

(the implementation is not actually done yet)
oob: Add some namespace constants
Add some structs for out of band data
form: Add namespace constant
form: Fix broken options
form: Factor out new field construction
form: Add unstable API warning to docs
form: Make Required show up under options in docs
Start outlining a potential data forms API
Make NS autocompletion easier in compression