Align respond_to? and method_missing in Matchers generated by DSL.

- Fixes #103.
changelog for #103
bump some dev dep versions and move them to gemspec
clean up file listings for rdoc, yard, and gemspec
ci: need the development group now
add README.md to .document file [ci skip]
remove unnecessary (and unused) custom error classes
Move all the matcher classes to the RSpec::Matchers::BuiltIn module (so
they don't cloud the Matchers namespace)
dev: tweak Guardfile
autoload matcher classes to reduce startup time
benchmark improvement using autoload
Changelog for #105 [ci skip]
fix test for cover
fix cuke that includes fully qualified name of a class that moved
it's cover? (with a ?)
Clear out user-defined instance variables between invocations of DSL-defined matchers.

- Fixes #104.
ruby 1.8 uses strings, 1.9 symbols
Perf: Set#include? is O(1); Array#include? is O(N).

See the included benchmarks for actual benchmarks against a collection of 7 values like we have here.
Dup the instance of a DSL generated matcher so its state is not changed
by subsequent invocations.

- Fixes #112.
Remove relative links from README, since they can't work correctly on
github and rubydoc.info.

- Fixes #113.