remove jqXHR property in cached ETags class.
add methods for get and set ETags caches, let user saving and restoring caches
removed _.extend in setCache
throw error if argument is invalid
fix mistyped argument name
no more underscore, anywhere
fixed a document bug logged as #26
Merge pull request #27 from Pomax/underscore-shim

no more underscore, anywhere
Merge pull request #24 from hyspace/master

Let user saving and restoring caches