Added boolean prompt type

[terrabitz]

Added ability to simply create boolean-based response (i.e. a 'yes' or 'no' answer). This is to simplify the common task of asking for y/n answers without needing to manually interpret the character. This also includes a default argument that allows a default response to be selected if there is nothing entered by the user. Otherwise, the prompt is displayed again. Here are several examples of this:

$>> boolean('Are you sure you want to update?', default=True)
Are you sure you want to update? (Y/n): 
True

$>> boolean('Would you like to delete the directory', default=False)
Would you like to delete the directory (y/N): y
True

$>> boolean('Should the new version be installed?')
Should the new version be installed? (y/n): 
Should the new version be installed? (y/n): y
True

[coveralls]

Coverage decreased (-4.9%) to 23.913% when pulling 68b645b on terrabitz:master into d2acf5d on sfischer13:master.