relocated ~/.srtm to ~/.cache/srtm

relocated cached SRTM files in order to match with debian home directory
recommended directory structure. see "B.2.2. The User's Home Directory"
http://debian-handbook.info/browse/wheezy/sect.filesystem-hierarchy.html
for details.
Merge pull request #7 from marook/cache

relocated ~/.srtm to ~/.cache/srtm