(piqi) Workaround for a Windows/MinGW/Cygwin runtime compatibility problem

When a MinGW build is called from Cygwin, both Sys.file_exists and
Sys.is_directory are not returning 'true' for directory names.
(piqi) Fix several source code source code location tracking problems

... in "piqi to-proto"
Bump copyright year
(erlang) Revert a no longer relevant example change
(piqi) Improve description of "piqi convert --gen-extended-piqi-any" option
(piqi) Better control over use of word literals in Piq

- generate a warning when string literal is used instead of word
- generate an error when verbatim text literal is used instead of word
- generate errors when word is used instead of string or text literals
Update documentation for 0.6.0 release
piqi-0.6.0 release
Bump version number to 0.6.0-dev