ipfs.files.add tests

[hackergrrl]

From @diasdavid: The tests should cover adding a files:

• file with less than a block size
• file bigger than a block size
• file bigger than a block size, but that is not a multiple of the block size (size % 256KiB !== 0)
• file that is 'big' (between 10 and 20Mb)
• directory with files
• directory with files and empty dirs
• directory with other directories that have files and other empty dirs (nested dirs)

[hackergrrl]

All of these tests now present in #20