Add the rbtools.api.cache module. This module includes the

APICache class, which allows the results of HTTP GET requests to

be cached locally to achieve faster lookup. The APICache understands

the following headers:

Cache-Control
Pragma
Vary
Last-Modified
ETag
Expires

The APICache is backed by a SQLite database in the user's cache

directory. If the cache cannot be accessed, the APICache will pass

all requests to the specified urlopen (defaulting to

urllib2.urlopen) method.
The APICache allows for a custom urlopen function so that it can

be tested without making actual HTTP requests.
The APICache stores the version of its schema in the database. If

the APICache is instantiated and the versions of the schema in the

class doesn't match the version of the schema in the database, then

the current cache will be invalidted and the schema will be updated.
Add the ActiveState appdirs module (at version 1.4.0) as

rbtools.utils.appdirs. This module is used to find the user cache

directory in platform-dependant ways.
Add the rbtools.api.cache.Response class which emulates the

behaviour of a urllib2 response. This class allows the read

method to be called more than once.
Add the rbtools.api.cache.CachedResponse class, which is the result

of a cach entry being found the the APICache. It also emulates the

behaviour of a urllib2 response.
Add the clear-cache command. This command manually clears the API

cache.
Added unit tests for the APICache.

Unit tests pass.
The following commands were tested and were successful:

rbt status
rbt post
rbt diff
rbt patch
rbt close
rbt clear-cache

Upon clearing the cache with clear-cache, a new cache is created.

The first run of a command always creates cache entries (where

appropriate, i.e., not for PUT or POST requests).
The /api/ endpoint is successfully cached and the cached version is

used.
When using the patch command for the second time on the same review

request, if the diff has not been updated, then the cached version of

the /api/review-requests/<id>/diff/<diff_revision>/ endpoint is

used.
This review request was posted with the APICache enabled.