add instrumentation for LDP operations
check if the resource exists by sending a HEAD request
Resource#reload should pass along the previous get response, so it can be checked against the current ETag/Last-Modified values
Merge pull request #20 from cbeer/head-etag

Head etag
Merge pull request #19 from cbeer/instrumentation

add instrumentation for LDP operations
LDP::Resource#create should accept a subject that is an existing absolute url within the LDP server