simplify the HttpParser interface
This cuts the HttpParser interface down to #execute and #reset
method. HttpParser#execute will return true if it completes and
false if it is not. http->nread state is kept internally so we
don't have to keep track of it in Ruby; removing one parameter
from #execute.
HttpParser#reset is unchanged.
All errors are handled through exceptions anyways, so the
HttpParser#error? method stopped being useful.
Also added some more unit tests to the HttpParser since I know
some folks are (rightfully) uncomfortable with changing stable C
code. We now have tests for incremental parsing.
In summary, we have:
* more test cases
* less C code
* simpler interfaces
* small performance improvement
=> win \o/