Added Attachments support for LRSs

[FishHooks]

Rebased with current master and fixed tests to support version 1.0.2

Addresses #70

This PR includes the following:

• Attachments can now be added to statements and sent to the LRS
• The attachments' content is sent with the statement but not serialized with the rest of the statement's JSON
• The LRS sends the put request in the correct format if the statement has an attachment (and the correct Content-Type)
• If the attachment flag is set to true in queryStatements(), the response is formatted correctly (previously queryStatements() did not handle multipart requests, and would therefore not return the attachments' content)

[brianjmiller]

It would be good to test the additional methods of LRS, and to have a test that sends an attachment, one that retrieves a statement via id, and one that query's statements.

[FishHooks]

This last commit adds tests for the new LRS methods, as well as sending and receiving statements with attachments.

Additionally, I've removed all manual header parsing and used the built-in methods of XHR to handle the response headers.

[brianjmiller]

Based on the other PR going in, this one needs to be merged/rebased with master again to resolve the conflicts. I'd probably lean rebase.

[FishHooks]

Alright. It's refactored, rebased, and retested. I ported the multipart response parsing helper methods straight over from PHP, so it should have everything. The only thing I removed was that PHP had it set up to put the status in the first element of the multipart sections array, so I removed that (I'm guessing it was there because of the way PHP handles the response).

[tseabrooks]

@brianjmiller We need to have this merged.

Thanks, TJ

[brianjmiller]

Closing in favor of #133.