Update NaCl FFI bindings to support constant-parameter autodetection

As of jedisct1/libsodium@98c02a libsodium has standard functions to
access configuration parameters that were previously only defined in
preprocessor macros. Since these are now available, we use them instead
of hard-coding the values in configuration.
Rename variables to increase consistency

The superclass that encodes the logic for a specific family of methods
in the NaCl bindings is referred to as a delegate in other areas, so
that name is now used consistently when it is stored in a
variable. Likewise, subclasses that inherit from the delegate are also
consistently assigned to appropriately-named variables.
Minor reorganization of methods for clarity
Update libsodium version
Include configuration entries missing from bf96a1b3
Rework automated testing setup

New features in this branch require an unreleased version of
libsodium. In the process of tweaking the build setup to use the latest
version on `master`, it made sense to *always* test against a local
build of libsodium.

This commit also cleans up the Rake tasks involved somewhat, and removes
a warning caused when building against newer versions of MiniTest.
Share data between rake tasks with ENV

Rake tasks can be loaded in any order, so don't depend upon global
variables which require specific ordering to be set. Environment
variables are a much better approach to sharing data between rake tasks.
Remove custom rake task for continuous integration

Since all testing now compiles a local copy of libsodium and loads the
library from it, there is no need for a custom task to do this for the
continuous integration server. It just happens automagically anyway.
Rename sodium rake tasks to libsodium

These tasks really have to do with the libsodium library, so should be
named accordingly.
Remove use of Enumerable#each_with_object

It isn't available in 1.8.7, so we can't use it here.
Allow configuration of libsodium mirror and version through env vars

This allows Travis builds against multiple supported versions of
libsodium (even though only the latest from master will work for now).
Remove use of Symbol#downcase for 1.8.7 compatibility
Merge branch 'master' into autodetect-sodium-constants

Conflicts:
	tasks/sodium.rake
Merge pull request #4 from stouset/autodetect-sodium-constants

Autodetect libsodium constants with library-provided functions