Adds a -R option to Snabb Switch which starts an interactive
Read-Eval-Print-Loop.
[bench_env] Use `run_nfv' where applicable.
[bench_env] Use Bash's `${VAR?}' syntax before using user
defined environment variables.
[bench_env] Pass nfv config to `run_nfv' in the individual
scripts.
Performance tweaks in core/timer.lua

The definition of call_timers() in core/timer.lua is moved out of
run_to_time() to avoid generation of the non-compileable bytecode
UCLO (bytecode 51 in the current version of LuaJIT).

Wrapping the call to ffi.C.get_time_ns() in run() with tonumber()
avoids the allocation of a cdata object that is difficult for the sink
optimizer to remove.
Merge pull request #337 from alexandergall/perf-tweaks-2

Performance tweaks in core/timer.lua
Merge pull request #334 from eugeneia/repl

Adds a -R (REPL) option to Snabb Switch
Merge pull request #328 from eugeneia/bench_env-regression

Fix bench_env regressions and improve error reports
[Makefile] [doc/genbook.sh] Simplify `genbook.sh' and make `make
doc/snabbswitch.md` work again.
[doc/] Minor fix ups in various documentation sources.
[genbook] Include version in metadata and only include authors
with full name (at least first+last name).
[make book] Remove epub task for now, make html task produce
self contained file which uses doc/style.css .
[documentation] Normalize typesetting conventions, add some
introductions.
[documentation] Repair pdf and epub targets.
[scripts/process-markdown] Center diagrams.
[doc/style.css] Center figures.
[documentation] Major overhaul.
[documentation] Document `apps/ipv6'.
[documentation] Document `apps/pcap'.
[documentation] Document `apps/vhost'.