Added modal close button in header by default

changed implementation so that by default the modal has header+close button and no footer
any header/footer element in the modal will override the default
adding data-header="none" will remove header by default
added panel class in modalContent to get the padding as other panels
added default modal demo
changed #header to .header to make it consistent
Merge pull request #553 from krisrak/2.1_modal

Added modal close button in header by default