rfc3: define matchtag as 32 bit integer

Redefine matchtag as 32-bit integer (was 8-bit).
This allows for more concurrency in clients.
rfc6: initial draft of RPC protocol spec

Also add a table of contents to README.adoc
rfc5: use relative link for rfc3 reference
Merge pull request #15 from garlick/rpc

rfc updates