Added bower reference to main less file

Myself, and many others, use grunt/gulp tasks to automate the injection of assets into index.html, unit test configurations, etc, by file type (see grunt-bower-install).  This same workflow is extremely useful for referencing external LESS libraries within our own less files so that we can reuse variables, mix-ins, etc, within our LESS.  However, we currently cannot do this with Bootstrap because bootstrap.less is not exposed.  By providing bootstrap.less as a main file with bower, we can facilitate this process and make developers' lives easier.