Fix typos in tests for mruby-string-utf8
Add Rake command 'deep_clean' which cleans everything including the mrbgems clone directories.
Implement :checksum_hash mrbgem option.

This allows to use a mgem with a specific checksum hash.
The following modification were necessary to implement this
function:
  - add run_checkout build command to use 'git checkout'
  - skip shallow copy in case checksum_hash is used
  - make 'master' the default branch if branch isn't defined
Symbol class include Comparable module.
String#casecmp should be call `to_str`
when non String object sent
And should be raise TypeError
when can not responsed to `to_str`
Implement Symbol#casecmp
Add AUTHORS "Yuki Kurihara"
Merge pull request #2084 from suzukaze/fix-typo

Fix typos in tests for mruby-string-utf8
Merge pull request #2090 from ksss/authors

Can I add me to AUTHORS?
Merge pull request #2085 from bovi/deep_clean

Add Rake command 'deep_clean' which cleans everything including the mrbg...
Merge pull request #2086 from bovi/checksum_hash

Implement :checksum_hash mrbgem option
Merge pull request #2087 from ksss/sym-comparable

Symbol class include Comparable module.
Merge pull request #2088 from ksss/string-casecmp

String#casecmp should be call `to_str`
Merge pull request #2089 from ksss/sym-casecmp

Implement Symbol#casecmp