Improve mirror selection process

[nroi]

The mirror selection process and the default settings should be improved such that:

• If the user has a high latency internet connection, the user will not end up without any mirrors (i.e., Flexo will not abort with the message Unable to find remote mirrors that match the selected criteria..
• Mirrors from different locations are considered so that users will run their latency tests against mirrors from the same continent (See Verbose mode? #22 for more details: The user is located in Australia, but the latency tests run only against European mirrors).

[nroi]

Plan so far: Implement two different strategies for selecting the mirror: cached and uncached.

• At the first start of Flexo, nothing is cached, so we use the uncached strategy: the number of mirrors to be tested is not restricted, all mirrors that match the criteria specified by the user (e.g. HTTPS supported, IPv6 supported) are tested.
• To avoid rerunning too many tests at each start, we store the country codes of the fastest mirror in a cache. For example, a user located in Germany could have the country codes DE, NL, CZ and FR stored in the cache. When Flexo is restarted, we use the cached strategy where only mirrors with the previously stored country codes are considered.
• After a specified time period (one week? one month?), the country codes are cleared from the cache and all mirrors are tested again.
• If the best mirror selected by the cached strategy exceeds a specified latency threshold, use the uncached strategy as a fallback (this should be useful in case the user travels from one continent to another and still has the cached results from the previous continent).
• Optionally allow the user to specify a list of country codes so that only mirrors from those countries are considered. This can be useful to decrease the startup time of Flexo and still get fast mirrors.

[nroi]

Fixed via #25
Fix is available starting from version 1.0.8