Added 'maximum number of selections' option

Limits how many options can be selected; default is no limitation
(null). Also updated the Readme file.
Readme Update

Updated readme to markdown style.
Fixed initialize _numSelected variable / 'None' button reset

_numSelected is now initialized with the number of preselected elements.
'None' button resets _numSelected to 0.
Fix initialize _numSelected & 'None' button reset

_numSelected is now initialized in createLis function. When the 'None'
button is pushed, _numSelected is reset to 0.
Merge pull request #5 from slimeygecko/master

Added 'maximum number of selections' option