Generate a random HTML string for data input.

Modified generate_string function to include an option that returns a
very simple HTML string. Right now the string generated is made up of
a single HTML tag surrounding a random alpha string. It would be nice
to eventually have a better generator to give us more 'advanced'
strings such as '<img src="http://example.com" alt="foo" />', but I
guess this is good for now.
Merge pull request #263 from omaciel/add-html-tag-generator

Generate a random HTML string for data input.
Updated generate_string function to better handle html output.

The new "html" output from the ```generate_string``` function will now
have lowercase tags, as this is the convention for writing html tags.
Also included "html" in the text displayed during an exception. This
fixes issue #264.
Merge pull request #267 from omaciel/issue-264

Updated generate_string function to better handle html output.