more progress on version update notifications getlantern/lantern#992

following 86a225f72, d2366d94, 1b860c83f, 63f1aee4, etc.

Replace dummy GET and PUT LanternVersionResource endpoints that
were exposed under /admin/rest/LanternVersion/{id}
with working versions exposed under /admin/rest/LanternVersion/latest
that read and update a singleton LanternVersion datastore entry.
Replace dummy LanternVersion.html UI with working form to update
the datastore entry.
improvements to SemanticVersion

- make the tag component (designates "pre-release" version in
  http://semver.org/spec/v1.0.0.html) optional as final releases
  won't have them
- make SemanticVersion comparable
- add some poor man's unit tests in a main method
temporary commit - @aranhoide will amend
first pass at version update notifications getlantern/lantern#992

goes along with getlantern/lantern@6d33036
Merge branch 'issue992/update-notifications' of https://github.com/getlantern/lantern-controller into issue992/update-notifications

* 'issue992/update-notifications' of https://github.com/getlantern/lantern-controller:
  temporary commit - @aranhoide will amend

Conflicts:
	src/main/java/org/lantern/XmppAvailableServlet.java
update to latest lantern-common
don't blow up when no version string is sent!
rm unused LanternControllerConstant LATEST_VERSION

stored in db, not hardcoded
update to latest lantern-common

adds SemanticVersion in getlantern/lantern-common@6393c44ae
SemanticVersion moved to lantern-common
ditch unneeded git sha field, improve error ux
ignore bin dir
nest version info inside a map so clients don't have to retrieve it by specific fields
ui improvements
refactor
fixes
Merge branch 'issue992/update-notifications'

* issue992/update-notifications:
  fixes
  refactor
  ui improvements
  nest version info inside a map so clients don't have to retrieve it by specific fields
  ignore bin dir
  ditch unneeded git sha field, improve error ux
  SemanticVersion moved to lantern-common
  update to latest lantern-common
  rm unused LanternControllerConstant LATEST_VERSION
  don't blow up when no version string is sent!
  update to latest lantern-common
  first pass at version update notifications getlantern/lantern#992
  temporary commit - @aranhoide will amend
  improvements to SemanticVersion
  more progress on version update notifications getlantern/lantern#992

Conflicts:
	src/main/webapp/WEB-INF/appengine-web.xml