Add BulkCreate function

[leeper]

CreateHIT isn't vectorized. Thus:

• [x] add a BulkCreate function that vectorizes it by taking a vector of question parameter values and return a dataframe.
• [x] create a wrapper function called BulkCreateFromTemplate that uses HITsFromTemplate to create a vector of question values, and then calls BulkCreate on that vector.
• [x] add documentation and update examples for HITsFromTemplate

[leeper]

Added documentation page to wiki: https://github.com/leeper/MTurkR/wiki/BulkCreate