Add public domain Maid helpers from @HazukiShiro

Public domain license notice located at
https://github.com/HazukiShiro/.maid/blob/master/helpers.rb

References #107
Refactor, document, and spec `dir_safe` and `downloading?`

Closes #107
Update mime-types to 2.0 -- just released
Merge branch 'stable'

Conflicts:
	ChangeLog
Merge branch 'stable'

Conflicts:
	ChangeLog
Merge branch 'stable'
Bump version to v0.5.0.alpha.1
(m) Update documentation to use GitHub repository (master branch)
Merge branch 'stable'

Conflicts:
	ChangeLog
	lib/maid/version.rb
Simplify definition of `find`, no need to be 1.8.7 compatible
Fix typos
Fix for Ruby 2.1.0-preview2

References #114
Fix obscure change to Dir.entries in Ruby 2.1.0-preview2

References #114
Note experimental Ruby 2.1.0 support, Closes #114
(m)
Tested on ruby-2.1.0-rc1
Note Ruby 2.1.0 compatibility
Bump version to v0.5.0