Hash passwords with BCrypt instead of SHA1

BCrypt is regarded as a more secure alternative to hashing using message
digest algorithms, such as MD5 and SHA families [0, 1, 2]. Apart from
built-in salting it is adaptable to the increasing power of modern
processing units, which makes it more secure against brute-force cracking.

This commit makes all passwords hashed using BCrypt. The session tokens
remain generated using SHA1. Tests were updated, `rake test:units` and
`rake test:functionals` didn't report any regressions.

[0] http://bcrypt.sourceforge.net/
[1] http://en.wikipedia.org/w/index.php?title=Bcrypt&oldid=439692871
[2] eab1c72/README.md

test/functional/users_controller_test.rb

@@ -68,7 +68,7 @@ class UsersControllerTest < ActionController::TestCase
     post :update_password, :updateuser => {:password => 'newpassword', :password_confirmation => 'newpassword'}
     assert_redirected_to preferences_path
     @updated_user = User.find(users(:admin_user).id)
-    assert_equal @updated_user.crypted_password, Digest::SHA1.hexdigest("#{Tracks::Config.salt}--newpassword--")
+    assert_not_nil User.authenticate(@updated_user.login, 'newpassword')
     assert_equal "Password updated.", flash[:notice]
   end