Fix longitude error for southern hemisphere.

The problem was the cprModFunction() not calculating the absolute value
of the latitude, so the table only worked for the northen hemisphere.

Thanks to @pwarren for finding the bug.

This fixes issue #13.
Fix incorrect extraction of vert_rate_sign.

See issue #26. Thanks to @vk1et for reporting and fixing.
Fixed typo in --help output for --enable-agc.

Thanks to Sanjeev Gupta for reporting.
Merge branch 'master' of git://github.com/antirez/dump1090