Make Locale objects immutable and cache them

[akx]

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Refs #31

[codecov-io]

Current coverage is 83.48%

Merging #305 into master will decrease coverage by -0.78% as of 0040a4b

@@            master    #305   diff @@
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  Files           22      23     +1
  Stmts         3776    3809    +33
  Branches         0       0       
  Methods          0       0       
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- Hit           3182    3180     -2
  Partial          0       0       
- Missed         594     629    +35

Review entire Coverage Diff as of 0040a4b

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[etanol]

Looks good. Although I can't risk my neck over the Memoized meta class, since I have played very little with them.

[akx]

@mitsuhiko: Would you have time to give this a look? :)

[akx]

This will probably need some rethinking wrt fuzzy locale support...