Added ability to resume download. Added support for Java 1.8.

[justinlittman]

Steps to test:

1. Replace settings/config.properties with config.properties from shared_configs. Set outputBaseDir to point to scratch space.
2. ./gradlew installDist
3. ./build/install/wasapi-downloader/bin/wasapi-downloader --collectionId 5425 --crawlStartAfter 2018-01-01
4. Download some files and then interrupt. Notice which files successfully downloaded and which is partially downloaded.
5. ./build/install/wasapi-downloader/bin/wasapi-downloader --collectionId 5425 --crawlStartAfter 2018-01-01 --resume`

Expected result:

• Partially downloaded file is deleted and downloading is repeated.
• Already existing files are skipped. For example:

  Cli-dl-7328-02d5:wasapi-downloader jlittman$ ./build/install/wasapi-downloader/bin/wasapi-downloader --collectionId 5425 --crawlStartAfter 2018-01-01 --resume
  file already retrieved: https://warcs.archive-it.org/webdatafile/ARCHIVEIT-5425-TEST-JOB555436-20180309195356729-00000.warc.gz
  file already retrieved: https://warcs.archive-it.org/webdatafile/ARCHIVEIT-5425-QUARTERLY-JOB694543-SEED1050159-20181027205452505-00001-h3.warc.gz