Update srtm.php

- fix srtm filename to always return positive number (eg S-33 -> S33)
- fix offset for Lontitude when South lat.
- remove duplicate lat/lon adjustment
- wrap long lines
Update srtm.php

lon_adj accuracy correction. validated against http://www.heywhatsthat.com/profiler.html
Merge pull request #59 from jammin84/patch-1

Update srtm.php