Enable some Xlint warnings

Also address a handful of warnings.
Reduce testFileGetParallel input size to ~160 KB

Previously this test downloaded 45 MB in 30 seconds which failed on
slower connections, causing spurious test failures.  The larger input
size provides no benefit.
Upgrade to Guava 15.0-rc1

Release notes:

https://code.google.com/p/guava-libraries/wiki/Release15
[trivial] Simplify use of Optional.or

Also correct checkArgument call.