Added a way to uninstall a language definition

moment.lang(key, null) now uninstalls a language. That's mostly useful to
remove auxiliary languages added during test runs.
string-driven getters and setters
AspNET duration regex now correctly handles days

Days are now completely optional and won't steal digits from the hours.
Merge pull request #910 from ichernev/bugfix/asp-net-duration-regex

AspNET duration regex now correctly handles days
Merge pull request #938 from icambron/string-driven-getters

string-driven getters and setters
Merge pull request #911 from ichernev/feature/uninstall-lang

Added a way to uninstall a language definition