Bumps version to v0.4.2.
Updates change log.

Merge branch 'release/v0.4.2' into develop
Fixes FileCache.__contains__ and __iter__ when sync is on

Internal method _all_keys tried to to access _buffer which is only available
when sync is off.
Merge pull request #19 from soult/develop

FileCache._all_keys always tries to use _buffer
Adds tests for #19.
Adds soult to AUTHORS.txt.
Bumps version to v0.4.3.
Bumps version to v0.4.3.
Merge branch 'release/v0.4.3'