gwc_config python 2.6 compat and bug fix

etree xpath failure under 2.6 and potential missing element
filter users from search follow up, refs #447

bug fix, add signal for avatar change, add warning to profile edit
filter users from search follow up, refs #447

avoid Contact model validation, set name field based on first/last
filter users from search follow up, refs #447

fix form bugs from last commit, fix avatar resolution, tests (duh)
use new thumb api to find if file exists, ref #503
Merge branch 'master' into production