add DictionaryExtension
add test
fix bug

When the value in a dictionary is null, returned None value.

fixed test case:
MaybeGetValue_NullValue
GetValue_NullValue
Merge branch 'master' into feature_dictionary_extension

Conflicts:
	Source/Maybe/Maybe.Test/Maybe.Test.csproj
	Source/Maybe/Maybe/Maybe.csproj
Merge pull request #8 from 100poisha/feature_dictionary_extension

Feature dictionary extension