Twitter Timeline: fix Undefined index notice
tweet-limit was undefined in defaults

Read more about the issue here:
http://wordpress.org/support/topic/undefined-index-tweet-limit
Twitter Timeline: add a message to explain 0 tweet-limit

See #6 for more details.
Revert "Twitter Timeline: add a message to explain 0 tweet-limit"

This reverts commit 2eedb2e8ed092f58f07f65daaaa92967a11bc776.
Twitter Timline: use null as default value for # of tweets
Null is the default value (unlimited amount of tweets)
Input now uses `number` type with `min` and `max` values as recommended in the twitter documentation:
https://dev.twitter.com/docs/embedded-timelines

See discussion in #6 for more details.
Merge pull request #6 from jeherve/tweet-limit-notice

Twitter Timeline `undefined index` & html5 input.
Migrate the tweet-limit to use input type="number" and set a min and max of 1 and 20, respectively.
Fix the undefined index by setting a default.