Added gpgme autoconf m4 macro to fix an undefined AM_PATH_GPGME error

For systems that don't have libgpgme installed, the addition of the m4/gpgme.m4
file fixes the following error when running the autogen.sh script:

configure.ac:313: error: possibly undefined macro: AC_DEFINE
      If this token and others are legitimate, please use m4_pattern_allow.
      See the Autoconf documentation.
configure.ac:326: error: possibly undefined macro: AM_PATH_GPGME
configure.ac:329: error: possibly undefined macro: AC_MSG_FAILURE
Merge branch 'master' into gpgme_autoconf_macro
added m4/gpgme.m4 file
Merge branch 'gpgme_autoconf_macro'

This commit adds a new m4/gpgme.m4 to allow autogen.sh to work properly when
libgpgme is not installed.  Closes #72.