Fix bug to support application/xhtml+xml

I tracked down this bug as part of debugging this issue: https://github.com/ankit/stylebot/issues/156

xhtml requires that all tags are properly closed
Updated measureString to return 0 for an empty string.
In autoGrow, track the input's current width in a closure rather than
using jQuery .width() to see if the width needs updating.
don't preload results in setup() when preload option is set to focus
Added links to the compiled Bootstrap 2/3 themes
Select option with TAB key

A client asked me they want to able to select an option by pressing tab. I thought it would be nice if Selectize has this feature by default.
Merge pull request #253 from Illianthe/master

Updated README
Fix for Bootstrap @dropdown-caret-color deprecation (fixes #232).
Document createOnBlur (#264)
Added a clearCache method to clear the render cache
Allow CTRL N/P to control down and up
Fixed compilation problems introduced by Bootstrap (now requires less >1.4). 
Mentioned in #282.
Merge remote-tracking branch 'refs/remotes/caseywebdev/ctrl-n-p'
don't trigger dropdown when creating item on blur

When using a multiple select with the createOnBlur option enabled, the
options list remains open when the control is blurred and a new item
is entered into the input. This patch adds a toggle to the createItem
method to prevent refreshOptions triggering a call to open()
remove_button: fix event handler when label contains html
Merge pull request #258 from code-guru/patch-1

Select option with TAB key
Merge pull request #267 from Deraen/feature/document-createOnBlur

Document createOnBlur (#264)
Merge pull request #245 from lananelson/fix-preload-on-focus

don't preload results in setup() when preload option is set to focus
Merge pull request #243 from enoshixi/master

Updated measureString to return 0 for an empty string.
Merge pull request #230 from ankit/patch-2

Fix bug to support application/xhtml+xml
Manual merge of #330 from @jkimbo w/o built files.