Replace httplib and urllib with httplib2

The module pywikibot.version contacts the code repository servers
to fetch version information.  It uses httplib and urllib instead of
pywikibot 2.0's pywikibot.comms.http package which uses httplib2
with additional functionality like proxy & user-agent config, and
httplib2 has the capability to set a debug level on all requests.

As the version module is typically the first network communications of
pywikibot, as it logs version information after argument handling,
it is highly desirable that there are not several different libraries
performing network requests, each with their own quirks.
By delay-loading the pywikibot http until the online version methods
are invoked, module loading order problems as avoided.

This also replaces the hard-coded incorrect user-agent which was
'SVN/1.7.5-pywikibot1' to now be 'SVN/1.7.5 Pywikibot/x' where x
is always the current Pywikibot release version.

Bug: 66102
Change-Id: I5fa5c776cfd4c3239eb7fdca24a381e8029b26f5
Merge "Replace httplib and urllib with httplib2"