Make ruby-termios work with Ruby/RubyGems 2.0.0

Using Ruby 2.0.0-preview2 (RubyGems 2.0.0), the gem was failing to
install with this error:

  ...
  ...
  installing default termios libraries
  /usr/bin/install: `./lib/termios.rb' and `/path/to/gems/ruby-termios-0.9.6/lib/termios.rb' are the same file
  make: *** [/path/to/gems/ruby-termios-0.9.6/lib/termios.rb] Error 1

The solution, as pointed to by Kouhei Sutou[1], was to move extconf.rb
and termios.c to a new ext/ directory. This is in a similar manner to
the BigDecimal bug #7344[2].

1 - https://github.com/rubygems/rubygems/issues/411#issuecomment-11153724
2 - http://bugs.ruby-lang.org/issues/7344#note-6

Tested-by: Mark Somerville <mark@scottishclimbs.com> [F17, MRI 1.9.3/2.0.0-preview2]
Signed-off-by: Mark Somerville <mark@scottishclimbs.com>