Fix several problems in parsing Piq variants

- parsing nested "non-terminal" variants (i.e. those without name label) was not
  progressing to the next available option
- unsigned integer option values were not handled correctly in some cases
Rework parsing of Piq sub-variants

It is now strictly DFS as one would expect and has a cleaner implementation.

DFS means that variant and enum options, including those of nameless
("non-terminal") sub-variants, are tried in the order they are mentioned in
their variant/enum definitions.
Internal improvements

Mostly related to pretty-printing (i.e. literal-preserving) mode for Piq parser.

Pretty-printing logic is completely separatated from "relaxed" Piq string
literal parsing.
Further improvements in Piq option parsing
Piq: parse integer-like literals as words with --piq-relaxed-parsing

Allow a string which prefix looks like a number (e.g. 1foo) to be parsed as a
word/string.

Without --piq-relaxed-parsing, it would be considered an invalid integer
literal.
Piq: fix handling of embedded modules

This fixes "unknown type" error when types are immediately reused in data
objects that follow the the embedded modules.
Remove deprecated pa_labelscope camlp4 syntax extension

It is not compatible with the newest 4.02.0 OCaml version.
Update INSTALL
Manually flush stderr on logging

Apparently, OCaml 4.02 canged behavior and doesn't do automatic line buffering
for stderr.
Rename directories in test/
Shorten file names in src/
Update TODO
Add opam file for pinning
Remove tests dependency on environment

Remove no longer needed setenv.sh and bin/piqi
Import deps/optcomp-1.6

This brings us conditional compilation.
Restore compatibility with OCaml 3.12
piqi-0.6.7 release
Bump version number to 0.6.7-dev
Remove camlp4 dependency from piqilib/META
Run travis-ci builds only for master branch