update readme

Thanks to @virtuallynathan.
Walk code, by Chris Dance (codedance).
Added IPv6 parsing support.

Note: I personally think we should be returning an IP{} type rather than
a string, but I don't want to break the external API.
Implemented retry support

UDP is connectionless and retry needs to be implemented in code. This
retry logic is similar to what you'll see in net-snmp and Snmp4J.
Increased read buffer size

We don't have correct read buffer handling. If a packet length exceeds
our read buffer, we'll only get a partial read and parsing will fail.
The correct logic would be to read more off the connection appending the
data.  Implementing this correctly is quite complex.  Added a FIXME, but
in the meantime we'll increase the size of the buffer to minimize risk.
Seed our random number

* Go's rand library seems a bit brain dead in this regard!
Added test to validate API consistency
Added generic end-to-end integration test + bugfix

* The generic e2e test should work against any standard SNMP MIB-2 host.
* The e2e test found a bug in the retry code. Connection refused errors
  were being incorrectly override as a timeout style error.
Documented new tests in README
Merge branch 'new_master'

Conflicts:
	README.mkd
	examples/walkexample.go
	marshal.go
	walk.go