Rename metric methods

Since we're having more resourcest that aren't metrics, replace the
generic methods names. Eg, `fetch` => `get_metric`. Add aliases and
deprecation notices to the old method names so everything still works.
Add namespaced methods to Librato module
Update specs to use un-deprecated metrics methods
Add specs for deprecated methods
Use special syntax for yard linking deprecated methods
Fix accidental replacement
Fix yard not being able to deprecate aliases
Separate update_metric[s] methods
Update examples to use get_measurements
Raise an error on missing arguments to #get_measurements
Update readme with new method names
Merge pull request #85 from librato/chore/deprecate-ambiguous-methods

Deprecate ambiguous methods