finish methods chapter section 1

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finish method chapter section 2

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finish method chapter section 3,4,5

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finish method chapter section 6,7

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finish method chapter section 8,9,10

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finish method chapter section 11,12

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Merge pull request #5 from endyul/master

complete chapter 14
finish method chapter section 13

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Merge branch 'master' of github.com:carfly/thinkpython-cn
Merge pull request #6 from xiehuc/master

finish chapter method
Free should be freedom not no charge.
成为 should be 称为。
a 的 is missing.
Merge pull request #7 from DeathKing/master

Some small fixed.
begin chapter 15 objects
complete 15.1