Address UglifyJS2 warnings

When minimizing bootstrap-select.js with UglifyJS2, the following warnings are emitted:

bootstrap-select.js(49,1,49,1): warning : Dropping unused function argument e
bootstrap-select.js(128,1,128,1): warning : Dropping unused function argument index
bootstrap-select.js(148,1,148,1): warning : Non-strict equality against boolean: != true
bootstrap-select.js(170,1,170,1): warning : Non-strict equality against boolean: == true
bootstrap-select.js(172,1,172,1): warning : Non-strict equality against boolean: == true
bootstrap-select.js(209,1,209,1): warning : Dropping unused function argument value
bootstrap-select.js(209,1,209,1): warning : Dropping unused function argument index
bootstrap-select.js(280,1,280,1): warning : Side effects in initialization of unused variable menuA
bootstrap-select.js(374,1,374,1): warning : Dropping unused function argument e
bootstrap-select.js(383,1,383,1): warning : Dropping unused function argument e
bootstrap-select.js(471,1,471,1): warning : Side effects in initialization of unused variable $this
bootstrap-select.js(527,1,527,1): warning : Dropping unused function argument e

This pull request addresses those warnings.

I realize a pre-minimized version of bootstrap-select is already provided.  However, with my current workflow, it is easier to let my build process automatically pick up and minimize files, rather than adding a special exception to use the pre-minimized version of this one file.  These warnings seemed to be reasonably easy to address, so I thought I might contribute a bit.