Improve input verification on publish template rule.

Change required by #42 to allow deferred initialisation of the credentials used
to publish templates. `PublishTemplateRule` now verifies that the upload tasks
themselves have the required properties set, not whether the extension does.
Remove build requirement for repo project properties.

Fixes #42. Any part of the build that needs Bintray credentials is now lazily
initialised. This means that build users that aren't publishing anything don't
need to set the required project properties.
Fix URL resolution in Bintray upload task.

The use of `URI.resolve()` meant that the task behaved differently when the
repository URL ended with a '/' compared to when it did not have a trailing
slash. The task now uses plan string concatenation with a '/' check.
Add IDEA and Eclipse Gradle plugins.