added gitignore

every project should have one of these. Ignores stuff in folders
which should never be checked in.
added support to build with gradle

in order to make the gradle support work you need to clone zxing into
the lib folder (so you wouldd have
libs/zxscanlib/AndroidManifest.xml

note that I have an outstanding pull request for zxing to add gradle
build suppert there as well
(https://github.com/LivotovLabs/zxscanlib/pull/1).
If you cannot wait you can clone https://github.com/rburgst/zxscanlib
instead.