Set platform to "java" under JRuby

Update the ObjC projects for Xcode 8

- Let Xcode 8 update settings on the projects/schemes.
- Migrate Swift tests to Swift 3 syntax.
- Update the build/test script:
  - Require Xcode 8 (because of the Swift 3 requirement for tests)
  - Update the devices to what Xcode 8 has (8.x simulator seem to fail even
    though they can be downloaded in Xcode 8)
- Update the travis images to ones with Xcode 8.

Updated .gitignore with Java and JavaScript build artifacts

Uses values from the removed `Gemfile.lock` as a baseline for version requirements, though it has been observed to work with the latest versions of all the referenced gems - see https://travis-ci.org/google/protobuf/builds/163625616 for details

Avoid [failures in travis](https://travis-ci.org/google/protobuf/jobs/163611664) under jruby

Proposed fix for #1594

Drop the swift docs directory (and content).

Since the official home is looking like it will be in another repo, it makes
more sense for the relevant docs to live there instead.

Merge 3.0.x into master.

Added convenient script for generating changelog draft.

Update README.md

Update README.md

gson 2.3 has internal bug that it doesn't work with some versions of maven.

Fixes static analyzer issues from xcode.

Remove obsoleted project of j2me

Fix hash computation for JRuby's RubyMessage

`System.identityHashCode` returns a hash that does not consider a
Message's values. This means two Messages with identical values will not
have identical hashCodes.

This patch uses the pattern from RubyMap to combine the hashCodes from
all values in a given message and produce a unique, consistent,
value-based hash.

Objc extensions tweaks

For the secondary dictionary, use a custom CFDictionary with integer keys to
avoid the NSNumber conversions.

Fix #2036 (Ruby `hash` broken for Messages with repeated fields.)

Fix erroneous comment regarding String.substring

Since Java 7, Strings produced with String.substring do not share byte arrays.

Some php engine implementation doesn't have return_value_ptr properly set. Explicitly use &return_value.

Fix the version number for 3.0.2

Merge 3.0.x into master

Update the ObjC version checks to support a min and current version.

- Capture the version used to generated.
- Check at compile time and runtime that generated code isn't from a newer
  version, also check that the min version required is also supported.
- Keep the old constants/macros/functions to special case the last version
  that was working so those generated sources still work until we decide
  otherwise.