generate etags with 'make TAGS'
replace hack using private header with public gpgme_signers_count()

This is how I originally figured out how to get it working, then Werner
added the gpgme_signers_count() function as of gpgme 1.4.3.

fixes #1549 https://dev.guardianproject.info/issues/1549
include data.h from gpgme 1.4.3

Currently, this needs stuff in the private gpgme header data.h, so we
include it here for now.  Ultimately this should be replaced by code that
only uses the public API in gpgme.h
update build version to 0.2-pre