Bug fix (maybe) and lowering the target API level

Hi I discovered your project from StackOverflow and really like it.

To compile your code, I had to made some bug fix in MainActivity and CustomGalleryActivity.

Rather than that, I lowered the target API level to 7, which may make MultipleImagePick used more widely, since lots of people still use old android system like 2.1.

If you don't like the lower of API level, please do check MainActivity and CustomGalleryActivity.

Thanks