Add "make" wrapper that runs gmake when available

It helps with building piqi-erlang as a rebar dependency on FreeBSD where GNU
Make is known as "gmake" instead of "make"
(doc) add a section about the new 'erlang-default' alias property
Update piqi_binary dependency to v0.6.5

Also, update (reboot) src/piqi_piqi.{erl,hrl} to the latest (v0.6.5) self-spec
that has support for the new piqi/file and field/json-omit-missing properties.
Add travis-ci build status image
piqi-erlang v0.7.0 release
travis-ci: build on R16B01, R16B02 OTP releases
Add new options for converting from/to Piq format

piq_frameless_output

     Print a frame (i.e. :<typename> []) around a single output Piq object
     (default=false)

piq_frameless_input

     Expect a frame around a single input Piq object (default=false)

piq_relaxed_parsing

     Parse Piq format using "relaxed" mode (default=false);

     For instance, when set to `true`, single-word string literals don't have
     to be quoted
Rename conversion option: json_omit_null_fields -> json_omit_missing_fields

... to better reflect the purpose.

The change is backward compatible as the old name is still accepted.
Add support for a new .piq-positional field property

Just pick up the new self-spec from the development version of piqi.

When parsing records represented in Piq format, it allows to explicitly control
whether to accept a field if it is specified without a label (arguments without
a label are called "positional").

The default behavior is to allow fields of primitive types to be positional but
require field labels for records and lists.
Merge remote-tracking branch 'anton/master' into bump_to_upstream

Conflicts:
	rebar.config
Ensure compatibility with piqi > 0.6.5

Piqi backwards incompatibly changed the command

        piqi version

to
        piqi --version

so checking of the version would fail for new versions.

This patch makes it work with both commands and also does proper version
parsing.
Merge pull request #4 from yfyf/bump_to_upstream

Bump piqi-erlang to upstream version