bnd.gradle build: Add description and group to doc tasks

Signed-off-by: BJ Hargrave <bj@bjhargrave.com>
gradle plugin: Use builtBy to connect artifact to task

This is more the "gradle" way and removes the need for the explicit
dependsOn from assemble to jar.

Signed-off-by: BJ Hargrave <bj@bjhargrave.com>
Merge pull request #720 from bjhargrave/gradle-changes

Gradle changes
gradle plugin: Add ignoreFailures flag to check task

This allows the user to configure a project with:

    check.ignoreFailures = true

to not fail the build if the check task fails.

See https://github.com/bndtools/bnd/issues/719

Signed-off-by: BJ Hargrave <bj@bjhargrave.com>