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.
Merge pull request #173 from CmdrMoozy/master

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