BOOKKEEPER-1054: Add gitignore file
BOOKKEEPER-1054: Add gitignore file

We should have a .gitignore file to hide all build generated files.

Author: Matteo Merli <mmerli@apache.org>

Reviewers: Jia Zhai <None>

Closes #144 from merlimat/git-ignore

ivankelly/bookkeeper,apache/bookkeeper,sijie/bookkeeper,sijie/bookkeeper,ivankelly/bookkeeper,sijie/bookkeeper,apache/bookkeeper,apache/bookkeeper,sijie/bookkeeper,sijie/bookkeeper,sijie/bookkeeper,ivankelly/bookkeeper,sijie/bookkeeper,sijie/bookkeeper,apache/bookkeeper,apache/bookkeeper,apache/bookkeeper