build.gradle: construct version from git

The `ext` trick allows us to memoize for all subprojects.

Example:

    % find . -name '*.jar'
    ./gradle/wrapper/gradle-wrapper.jar
    ./plog-api/build/libs/plog-api-4.0.0-ALPHA-5-g30c49eb.jar
    ./plog-console/build/libs/plog-console-4.0.0-ALPHA-5-g30c49eb.jar
    ./plog-distro/build/distributions/plog-distro-4.0.0-ALPHA-5-g30c49eb-shadow.jar
    ./plog-distro/build/libs/plog-distro-4.0.0-ALPHA-5-g30c49eb.jar
    ./plog-kafka/build/libs/plog-kafka-4.0.0-ALPHA-5-g30c49eb.jar
    ./plog-server/build/libs/plog-server-4.0.0-ALPHA-5-g30c49eb.jar

Closes #51.