server: Rename address environment variable

To follow the pattern of using NVIM for technical descriptions, it was renamed
to NVIM_LISTEN_ADDRESS
eval.c: Rename jobwrite to jobsend
eval.c: Rename msgpack-rpc functions for consistency
runtime: Enhance help.vim to highlight Nvim/NVIM
runtime: Initial msgpack-rpc documentation
runtime: Initial job control documentation
runtime: Add highlighting for new functions/events
runtime: Update python_setup.vim to the new rpc function names

Also improve version checking.
runtime: Document 'unnamedclip'
runtime: Document python interface, clipboard and add intro doc
Merge PR #1163 'Initial documentation of nvim features'