Add support for SINCEDB_DIR to file input

This change allows you to change the destination of sincedb files without having to change the $HOME environment variable.  If set, SINCEDBB_DIR will override HOME as the destination of default sincedb files.
Added a test for the SINCEDB_DIR functionality

Setting SINCEDB_DIR in your environment allows you to override the target directory for sincedb files.  The added test verifies that this functionality works properly
Merge pull request #525 from tejaycar/master

Allow custom global sincedb target directory