Require Redmine 2.1+

Update Redmine required version to 2.1+ to be consistent with README.md.

Version 2.1+ is required since it introduced jQuery instead of prototype.

This closes #22
Translated to Bulgarian:
* issues_use_colors
* issue_tags_sidebar_simple_cloud
typo
add for attributes on labels in tags/settings view
update readme for wiki page tagging
Merge pull request #27 from jwalkerbg/master

Translated to Bulgarian language
Merge pull request #29 from hdgarrood/update-readme

update readme for wiki page tagging
Merge pull request #21 from Cantemo/master

Function names for sidebar tag list collide
Merge pull request #28 from hdgarrood/associate-labels

add for attributes on labels in tags/settings view
create a new helper instead of patching IssuesHelper

partially fixes #17
refactor WikiHelperPatch into WikiControllerPatch + WikiTagsHelper
add TagsHelper to IssuesController
remove unnecessary require_dependency
fix NoMethodError in views
fix more NoMethodErrors
Merge pull request #23 from hdgarrood/fix-helpers

Fix multi-plugin 500 errors
Include issue tags helpers for gantt and calendar

Fixes #33.
Also added a GPL notice and refactored a teensy bit.
Merge pull request #34 from hdgarrood/issue-33

Include issue tags helpers for gantt and calendar
Fix copying and remove history
Fix tests (#47)

* Fix main test_helper path
* Replace removed method generate_for_project! with generate!
* Use Rails fixtures to ensure data is the same for test runs
* Fix assertion (there should be 5 available open tags, not 4)