ignore config.cache/test-driver files

config.cache is generated when you run `./configure -C`.

test-driver comes from newer autotools.
add a --disable-samples configure flag

For people building & installing libevent in a distro, building all these
sample apps are not useful.  Add a flag to turn them off.
Merge pull request #143 from vapier/disable-sample

add a --disable-samples configure flag
Merge pull request #140 from vapier/ignore

ignore config.cache/test-driver files