Expand functionality

[Szelwin]

I've been using this lib for a while now, adding stuff whenever I had the need. Added functionalities:

• Check how much space is used and how much is available
• Reset the buffer
• Peek backward and forward to allow analysis without touching the rw indices
• Locate the buffer in a specific section of memory with the help of a linker script

Also added tests for this (except for the linker section allocation - all I can do is confirm it works for me ;)) and updated the README.

Thanks for your great work, @pthrasher :) works like a charm.