update libcityhash to 1.1
test against ruby-head
add .hash32 method
version bump
update travis file with ruby 2.0
update minitest
add crc methods
add crc256 method
bump version
compile with SSE4.2 only if it's available

closes #18
always convert input to string
add ability to specify c++ compiler
update README
update CHANGELOG
bump to 0.8.1
update city.cc to latest version

closes #22
Validate hash arguments