Use appropriate URIs for From, To, Route, and Record-Route headers

In To, From, and Contact headers, a conforming endpoint should
consider parameters in a URI without angle brackets to be header
parameters.

Route and Record-Route header are specified as being `name-addr'
rather than `name-addr / addr-spec`.  As such these should _at least_
be wrapped with angle brackets.

Potentially there is room to make the client more user friendly in
this respect, but for now, correctness is probably more important.
Fix parsing of SIP URIs that are not enclosed within angle brackets

These following the rules in Section 20 of RFC3261 where parameters
are treated as header parameters rather than URI parameters when not
enclosed by angle brackets.

The primary symptom was that the 'tag' parameter was ending up in
#uri.opts rather than #uri.ext_opts.
Merge pull request #8 from danielwhite/use-valid-uris

Fix the parsing of addr-spec based URIs in headers