Add Gemfile.lock and sort .gitignore

Closes #179 [ci-skip]
Add requirements section to README

This includes a new admonition against using Ruby 1.8.7.

[ci skip]
A future file jumped the gun :-)
Changed use of spec/base.rb into spec/spec_helper.rb
Reorganize specs

* Put unit and integration specs into separate subdirectories
* Consolidate all requires of 'webmock/rspec' into spec_helper.rb
* Use WebMock.{disable!,enable!} for specs calling live server
For streams, actually use a stream

This avoids loading all of the data to be transmitted into memory
Get tests working with payload-requiring Request::transmit