Port over a bunch more transcoding stuff + gzip encoding logic.
Patch up things that broke from my mega-port.

* GzipReader#read always returns ASCII-8BIT strings when given a
  length.
* String#encode needed some ported logic to fit other pieces.
* Get ECONV constants propagated into RubyConverter.
* Encoding 7-bit strings from/to ASCII-compat encs is a no-op.
Large port of MRI IO transcoding logic, to fill in our gaps.
Fixes and a few additional ports to get test:mri19 green.

Additional fixes to get test:mri18 green.
Edging closer to everything passing again.
rspec let to stop repetation of same code

using let of rspec for non repetative declaration
Merge pull request #947 from rajcybage/fix_spec

rspec `let` to stop repetation of same code
Revert "rspec let to stop repetation of same code"

This reverts commit f6318f6e60b7cc317c65658063db5fbeb5130848.

We should test both `:str_field` and `:strField`.
Fixed javadoc for getRubyCasedName
Use normal "new" logic in Regexp. Fixes #955.
Set `jruby.jit.exclude` method to `''`.

The old default `none` prevents a method named `none` to be
JIT-compiled.

Fixes #954.
Remove mention of .jrubyrc for C Extensi building [skip ci]

Fixes #953
Enabling C Extensions via `.jrubyrc` is not effective for building gems,
so remove the mention of it to avoid confusion.
Change `--properties` output for null default opts [skip ci]

Fixes #949

For options whose default is `null`, it is confusing to print out the
string `null`, since it is not a likely value that the user can use.
While the message makes it clear that the value will be interpreted as a
String, it is a good idea to leave it blank instead, so that the user
can fill it themselves.
added some note about maven version
fix parent-pom for jruby-jars
added snapshot repo for plugins
added missing dep to reactor
expose JRubyJars::VERSION with loading gem
Use 'let' in code, and fix spec description

Avoid hard coding reused value.
Fix spec description text