Replace "mock" calls with "double"

The "mock" method was deprecated in rspec-mocks 2.14.0.
Drop support of Ruby 1.8.7, add support of Ruby 2.0.0
Implement incremental logging

Before this commit, the input consumed by the executed command and the
output produced by it were logged as a whole when the command finished.
Now they are logged incrementally one-by-line as the command executed.
This should help debugging long-running commands.

Note this change alters the semantics of the following methods:

  Recorder#record_stdin
  Recorder#record_stdout
  Recorder#record_stderr

Before this commit, they were called only once in a certain sequence and
they were passed the whole input/output. Now they can be called
repeatedly anytime after Recorder#record_commands and before
Recorder#record_status. Each time they may be passed only a part of the
input/output.
Improve DefaultRecorder spec descriptions

Mostly s/record/log/. We test that the recorder *logs* into the log.
Update CHANGELOG
Update version ot 0.4.0