Allow transparent use of Pathnames

Pathnames can now be used:

* In `file` and `directory` tasks
* As prerequisites of tasks
* In FileLists, as `include` and `exclude` patterns, and also with the
  `<<` operator

There is a new method on the Rake module called
`from_pathname` (better name?) that calls either `to_path` or `to_str` on its
argument; if neither of those methods are defined, it just returns the
argument directly.  `to_path` should be sufficient, but `to_path` was only
implemented in Ruby 1.9, so `to_str` is there for 1.8.7 compatiblity.

The `from_pathname` method uses a similar mechanism to that used by
`File.open`, as described by Avdi Grimm in his excellent book,
[Confident Ruby](http://www.confidentruby.com/).
Support the `ext` and `pathmap` extensions on Pathnames.

These extensions are not loaded automatically, because they require 'pathname'
to be loaded, and I don't want to force that upon Rake users.  Ideally, there
would be a way to automatically define Pathname extension methods after
the Pathname class has been defined, but I couldn't find a reasonable way
to do that.

So, in order to use Rake's extension methods on Pathnames, the client code
must require 'rake/ext/pathname' explicitly.
Ignore falsey dependencies

When creating Rake tasks programatically, it could so happen that the
prerequisites reach the point of creation as an argument. This change
teaches the Rake DSL to ignore falsey (`nil' or `false') values when
they are specified as the dependencies; this way, programmatic creation
does not have to deal with a special case.
FileTask: Return a late timestamp instead of an early timestamp if the
file does not exist.  This should fix issue #286.
Update our monkey patch of Time#<=> to consider Rake::LateTime.
This makes the tests pass on Ruby 1.8.7.
better manpage
removed old manpage
Merge pull request #291 from skittleys/master

Updated and improved manpage
Add #291 to History
Merge pull request #287 from DavidEGrayson/pull_request_1

FileTask: Return a late timestamp instead of an early timestamp if file doesn't exist
Add #286, #287 to History

Fixes #286
Upgrade minitest
Replace `MiniTest::Unit::TestCase` with `Minitest::Test`
Merge branch 'teoljungberg-minitest-5'

* teoljungberg-minitest-5:
  Replace `MiniTest::Unit::TestCase` with `Minitest::Test`
  Upgrade minitest

Fixes #292
Add #292 to History
Merge pull request #271 from randycoulman/support-pathname

Support Pathname
Add #271 to History
Merge pull request #273 from mx4492/ignore-falsey-dependencies

Ignore falsey dependencies
Add #273 to History
Update Manifest