Provide has() method to Document
Add simple tests for has() method
Merge branch 'has_field'

* has_field:
  Add simple tests for has() method
  Provide has() method to Document

Conflicts:
	tests/Prismic/DocumentTest.php
Build <img> tag programmatically
Add alt and copyright fields to images in test JSON, as per #23
Store and expose image alt and copyright info
Include image alt in markup
Add ImageView tests