Allow specifying Bundle-RequiredExecutionEnvironment header via setting. Automatically include default headers which have values defined in build. 

- Adds `bundleRequiredExecutionEnvironment: SettingKey[Seq[String]]` for including `Bundle-RequiredExecutionEnvironment` manifest header if not empty. Sets default to `Nil`.

- Includes `Bundle-Description` manifest header set to the value of `Keys.description` if not empty.

- Includes `Bundle-DocURL` manifest header set to the value of `Keys.apiURL` if set.

- Includes `Bundle-License` manifest header set to the head value of `Keys.licenses` if not empty.

- Includes `Bundle-Name` manifest header set to the value of `Keys.name`.

- Includes `Bundle-Vendor` manifest header set to the value of `Keys.organizationName` if not empty.

- Fixes scripted tests broken by #18. (Obsoletes #19)

- Updates unit & scripted tests for new functionality.