Restricts permission to activeTab and pinboard

Hey, thanks for writing this extension. I really liked it, but wanted a few extra features so I went ahead and implemented them yesterday. 

This commit changes the permissions asked by the extension to just the active tab and https://pinboard, instead of the scary all-data-on-all-websites. The current text selection is now fetched by injecting a script [to just the active tab] through chrome.tabs.executeScript, instead of using content_scripts. 

I made a few other commits (that I haven't included in this request) that add the following features:

* Add the current text selection in the description field for read-later.
* Option to automatically fill in popular and recommended tags when marking a page as read-later.
* A keyboard command to save the current page without opening up a popup (basically, read-later without setting the toRead field. I wanted an easy alternative for Chrome's Ctrl+D shortcut that saved directly to Pinboard.)
* A better way of using chrome.tabs.executeScript for getting the tab details, instead of loading up a different file.

Let me know if you're interested; I'll send a pull request for them as well.