Contains key check for Contexts in GetModel.cs

Doing Context.Contexts[ContextName] results in an exception thrown by the dictionary and gives poor feedback about the missing context.
Merge pull request #37 from elwinarens/patch-1

Contains key check for Contexts in GetModel.cs
removed sc upgrade tests from 4.5 build
flag to check for page edit mode
page editor support only for fields
Merge branch 'develop' of github.com:mikeedwards83/Glass.Mapper into develop

Conflicts:
	Sites/Sitecore/Glass.Mapper.Sites.Sc.Tds/Glass.Mapper.Sites.Sc.Tds.scproj
fix to Editable method so context is passed in.
created override to turn of page editor changes
config file didn't get check in
Merge branch 'hotfix/PageEditorInNewLanguage'
Merge branch 'hotfix/PageEditorInNewLanguage' into develop
Merge branch 'release/20140130'
nuget updates
Merge branch 'master' of github.com:mikeedwards83/Glass.Mapper