Explicitly set box-sizing for .pika-button's.

If Pikaday is used in some environment that sets box-sizing to something
other than border-box, elements which use .pika-button are rendered
improperly. This commit adds an explicit box-sizing: border-box;
statement to this class to hopefully override some other inherited
box-sizing.
Revert "Replace data URIs with font arrows."

This reverts commit 40956076ad6b99b2fd43b87ec005711a1325b602.
Merge pull request #173 from CmdrMoozy/master

Explicitly set box-sizing for .pika-button's.
Add LICENSE file to simplify approval at big companies
Merge pull request #184 from saw/license-file

Add LICENSE file to simplify approval at big companies
Clear the <input> value when setDate is passed a falsy value