Update BoxSDK.h docs to refer to Parallel OAuth2 and queue implementations

[BoxSDK sharedSDK] is configured with the parallel varieties and has been since the
inclusion of 09f1d69126f04c3718dd7a2bea15965faf6b7cfb
run chmod -x on some source files that were erroneously executable
Document that upload and download operations cannot be automatically retried

Upload and download SDK operations hold references to NSStreams for input and output.
NSStreams are not copyable so neither are these operations. The SDK relies on copying
operations to reenqueue them after an OAuth2 refresh event.

A common scenario when tokens need to be refreshed is when an app is first launched.
In this case, it is recommended to issue a heartbeat API call to a well known resource,
for example, the root folder, to ensure that tokens are refreshed before initiating an
upload or download.
Document that the SDK uses its own OAuth2 redirect URI

Since the SDK uses its own custom redirect URI to facilitate returning to your app,
this field must be left blank on the Box developer settings page for your service.
Add missing docs for retryWrite: in BoxAPIMultipartToJSON.m
Merge pull request #17 from box/docs/fixes

Documentation improvements