Fix read-after-write

Fixes this problem:
https://forum.pjrc.com/threads/33594-SerialFlash-filesystem-question?p=101511&viewfull=1#post101511

(inportant, i think...)

Add readSerialNumber(uint8_t *buf);

Size of buf: 8 bytes

https://forum.pjrc.com/threads/33328-Prop-Shield-Beta-Test?p=98553&viewfull=1#post98553

Fix reading of filenames with length = multiple of 16

fix string_length

Update Readme for File Erase

...i don't know where this caused problems, but unpatched, it is wrong :)
I saw this when searching an other bug..

Bugfix blockSize (Spansion chip, model with 256K sector size)

The Spansion cards can have 64Kb or 256Kb depending on the model's
number. The device id[4] must be checked to determine the sector
architecture.

https://forum.pjrc.com/threads/27409-Play-RAW-from-Serial-Flash?p=91876&viewfull=1#post91876

Create library.json

Bugfix Capacity (important)