Add new ParseFile download api

[wangmengyan95]

Support fetch a ParseFile as File pointer or InputStream.
File related new apis:

1. public File getFile() throws ParseException
2. public Task<File> getFileInBackground(final ProgressCallback progressCallback)
3. public Task<File> getFileInBackground()
4. public void getFileInBackground(GetFileCallback fileCallback, final ProgressCallback progressCallback)
5. public void getFileInBackground(GetFileCallback fileCallback)

DataStream related new apis:

1. public File getDataStream() throws ParseException
2. public Task<File> getDataStreamInBackground(final ProgressCallback progressCallback)
3. public Task<File> getDataStreamInBackground()
4. public void getDataStreamInBackground(GetDataStreamCallback fileCallback, final ProgressCallback progressCallback)
5. public void getDataStreamInBackground(GetDataStreamCallback fileCallback)

[grantland]

nit: newline

[grantland]

Some nits and a thought Q

Do you think the getDataStream APIs are worthwhile when we have getFile APIs?

I think we should update the get(Data|File|DataStream) API documentation to match iOS's since it's a bit more descriptive:

getData:

Synchronously gets the data from cache if available or fetches its contents from the network.

getDataStream:

This method is like getData but avoids ever holding the entire PFFile contents in memory at once.

[wangmengyan95]

For getDataStream, this method is not like iOS which does not wait download to cache file finish but open a stream from the cache file directly. In Android, it just wait download finish and read the data from cache file for developers. So I do not hold strong opinions for either sides.

[grantland]

We should add the following:

<strong>Note: </strong> The {@link File} location may change without notice and should not be stored to be accessed later.

/cc @nlutsenko @richardjrossiii

[nlutsenko]

👍

[wangmengyan95]

👍

[grantland]

LGTM, feel free to squash and merge!

[facebook-github-bot]

@wangmengyan95 updated the pull request.