Right-click to add attachment

[stakats]

Right-clicking (or ctrl-clicking on Mac) could bring up a "Create Zotero Attachment from Link." Encouraging this type of user behavior will greatly simplify future translator writing. I'm not sure that we want to encourage the automatic downloading of entire PDFs, etc., when many users might only want to capture metadata. By adding the suggested add attachment functionality, users can add PDFs or whatever relevant linked files they want. Could also add "Create Zotero Attachment from Image" for with similar functionality.

[dstillman]

(In [924]) Closes #416, Right-click to add attachment

Added "Save Link As Zotero Snapshot" and "Save Image As Zotero Snapshot" options to the browser content context menu where appropriate

Other fixes:

• Implemented standalone image and plugin snapshots the right way (as opposed to the fairly broken way from yesterday)
• Only natively handled files are loaded into a hidden browser when using importFromURL() -- plugin files are now saved directly with saveURI()
• indexDocument() doesn't try to index non-text files

Notes:

• There's no feedback when saving large files, which will likely be a bit confusing for users -- one option would be to put the transfer into the downloads window, though that's a little weird.

• I suspect this will fix the reported JSTOR PDF download issue (​http://forums.zotero.org/discussion/217/), though I don't currently have a way of testing it.