use dict.get() call when retrieving props form page

[rowan08]

Use dict.get() method to avoid KeyError when a page has no external links. The WikipediaPage.references property will return a blank list instead when this happens.

Fixes issue #93

[TsheringL]

Use dict.get() method to avoid KeyError when a page has no external links. The WikipediaPage.references property will return a blank list instead when this happens.

Fixes issue #93

The above mentioned solution doesn't fix the keyerror.