Use fuzzy matching when trying to group albums during import.

(From the discussion in #764 )

When trying to group albums during import, beets uses the current values in `album` and `artist` fields as a grouping criteria, which causes problems with files having inconsistent metadata (different spelling for example).

Fuzzy matching could solve these problems.

Some example test cases which should be resolved (The artist column might refer to whichever one from `albumartist` or `artist` is present):

| Track | Artist | Album |
| --- | --- | --- |
| 01 | Alpha | beta |
| 01 | Gamma | delta-2010 |
| 02 | gaMMa | DELTA(xclusive!) |
| 02 | aLphA ft. GAmmA | BETA(www.example.com) |
| 03 | gaMMa featuring Alpha | Dleta |
| 03 | ALPHA | ~!Beta!~ |

The above tracklist should get grouped like this:

| Track | Artist | Album |
| --- | --- | --- |
| 01 | Alpha | beta |
| 02 | aLphA ft. GAmmA | BETA(www.example.com) |
| 03 | ALPHA | ~!Beta!~ |

| Track | Artist | Album |
| --- | --- | --- |
| 01 | Gamma | delta-2010 |
| 02 | gaMMa | DELTA(xclusive!) |
| 03 | gaMMa featuring Alpha | Dleta |