Included sieve plugin into the build process.

configure.in

src/Makefile.am

src/lib-sieve/sieve.c

src/lib-sieve/sieve.h

src/plugins/lda-sieve/Makefile.am

src/plugins/lda-sieve/lda-sieve-plugin.c

src/plugins/lda-sieve/lda-sieve-plugin.h

src/sieve-bin/bin-common.c