Use a properly formatted hidefocus attribute

If the document was served with Content-Type application/xhtml+xml,
browsers require the document to be valid XML. Without this commit
SlickGrid generates XML parsing error and creation of the grid will
fail.
Properly quote attribute values
Bugfix init Jquery UI Sortable Widget +1.9
fixed typo. backround to background in style definition
Added more information to the grid onCellChange event args
Fix Safari 6 toString() defaultFormatter() bug

On Safari 6 in Lion, the toString() method will fail on numeric data if
the Error Console is not open. By using string concatenation, this gets
around this strange Safari bug.
Lets ignore .DS_Store Files
removed MAC-specific cruft file
gitignore editor bak files to prevent clutter
added API getHeadersColumn() a la getHeaderRowColumn() so we can get a handle on each of the topmost header columns.
Merge branch 'Celebio-excel-import-export': included work for https://github.com/mleibman/SlickGrid/pull/359
Merge remote-tracking branch 'remotes/sfarthin/master': ignore DS_Store MAC file via gitignore

Conflicts:
	.gitignore
Merge remote-tracking branch 'remotes/tjormola/master': properly quote HTML attributes for XHTML compliance
add .onColumnsResizeStart event which signals the start of a column resize drag operation, next to the existing onColumnsResized event which signals the end of a column resize drag operation.
stricter checks in a few places to prevent crashes due to invalid row/cell indices, e.g. when addressing headerRow cells: catch NaN, undefined, etc. row/cell values by inclusive checks instead of exclusive checks.
added jQuery event.drag/drop as submodule + debugging code to track ddown focus/click problem in slickgrid + fix in event.drag defaults to ensure it does NOT steal the click event from the regular setFocus/click handlers: see event.drag submodule commit SHA-1: dbd4dc66b3ab3ac8d5f697c8145ea6f0d754d511 * prevent event.drag from stealing the mouseclick from other handlers: only hijack the event after the mouse has moved at least 1px.
revert debugging console.log lines from previous commit.
prevent JS crash when trying to decorate header cells, as those would produce from/to == null as .fromRow/.toRow would be < 0. Needs further work; hard to reproduce.
robustness against some editors, missing editors and minor bug fixes
Delete .DS_Store, create README