gradle plugin: Clean up export.* rule

Improved description and location of output.

Signed-off-by: BJ Hargrave <bj@bjhargrave.com>
gradle plugin: Add runbundles.* rule and runbundles task

New exportRunbundles method is added to Project in support of this new
gradle task.

Fixes https://github.com/bndtools/bnd/issues/541

Signed-off-by: BJ Hargrave <bj@bjhargrave.com>
Merge pull request #630 from bjhargrave/issue541

Add runbundles.* rule and runbundles task