Add cache for intersphinx inventories

[adiroiban]

Sometimes the intersphinx inventories are down

In this case, it would help to use a cached version, rather than to generate a documentation with invalid links...

The cache can have an expire age (ex 1 week), to prevent the case when the documentation was permanently moved or is permanently down

The cache can be placed in ~/.cache/pydoctor so that we can persist the cache in Travis-CI

We can start with Linux/Unix only cache support ... but maybe is easy to put it in APPDATA on Windows

There can also be a flat to disable cache --cache-age=0 so that at a release we are forced to check the state of external api docs.

---

We use pydoctor as part of the Twisted's test suite and I hope that with the cache we can prevent false failures due to temporary unavailability of a API doc site.

[adiroiban]

maybe use requests and then use https://pypi.python.org/pypi/requests-cache