Encode URL strings using URI's toASCIIString() method.

This makes Unicode in paths work correctly when the Java system
encoding isn't set to UTF-8.

Thanks to Marcel M for reporting the issue.
DbxEntry/DbxHost: Fix equals/hashCode/Serializable issues reported by FindBugs.
StringUtil: Get rid of public static mutable state.
DbxRequestUtil: Delete unused parseAsQueryString.

Was used for OAuth 1 response, but we don't support OAuth 1 anymore.

Also, FindBugs reported that it was using the system encoding instead
of a fixed encoding.
pom.xml: Add FindBugs plugin.
pom.xml: Move Checker Framework stubs to "support/" subfolder.
Convert code to use mkAssert helper.