reader: explicitly check that the input has at least MTBL_TRAILER_SIZE bytes

this ought to fix debian bug #716628.
src/test-* -> t/

move the test source files from the src/ directory to the t/ directory.
Makefile.am: minor readability tweaks
Makefile.am: tests have been moved into the t/ subdirectory
test-deb716628: new

this adds a test case based on debian bug report #716628.
update gitignore files for the tests
split out gitignore files for mtbl/, src/ directories
configure.ac: bump version to 0.5
ChangeLog: 0.5