Add Dallas DS1307 RTC protocol decoder

Dallas DS1307 RTC protocol decoder that works stacked
with the I2C PD. Based on the rtc8564 protocol decoder.

Signed-off-by: Matt Ranostay <mranostay@gmail.com>
ds1307: Fix date/day-of-week printing.
README: Refer to INSTALL and new Building wiki page.
configure.ac: Drop std-options, unneeded.

The automake 'std-options' option checks whether all installed tools
and scripts have a --help and --version CLI option. This check is not
needed for libsigrokdecode though, since it doesn't install any tools.
code cleanup
use GVariant for passing decoder options and probes

Decoder options are now exposed in struct srd_decoder, via a GSList
of the new struct srd_decoder_option. The 'def' field represents
the default value as set in the decoder, converted to GVariant. This
allows a library user to determine the type as well as default value.
README: Add "Copyright and license" section.
configure.ac: Look for python-config-3.x besides python3.x-config.
configure.ac: Show $build, $host, and lib versions.
GPL headers: Use correct project name.
Update package version to new 0.2.0 release
Doxyfile: Set version number to 0.2.0.
HACKING: Document the @since Doxygen tag.
Doxygen: Add @since markers to API functions.

Also, document the functions in version.c.
Main header is <libsigrokdecode/libsigrokdecode.h> now.

This matches the convention used in libsigrok. Potential other headers
might end up in libsigrokdecode/, but only libsigrokdecode.h is meant
to be #included by frontends directly.
Doxygen: Various fixes.
NEWS: Minor updates.
libsigrokdecode-internal.h: Cosmetics.
Add srd_strerror() and srd_strerror_name().
Use enums for error codes and loglevels as in libsigrok.