Merge pull request docker#2152 from nathanleclaire/bump_0.5.0

Bump to v0.5.0

Merge pull request docker#2120 from nathanleclaire/bump_0.5.0_rc4

Bump to RC4

Merge pull request docker#2049 from nathanleclaire/0.5.0-rc3

Bump version to rc3

Merge pull request docker#2015 from nathanleclaire/bump_0.5.0_rc2

Bump version to 0.5.0-rc2

Merge pull request docker#1997 from nathanleclaire/bump_for_rc

Bump version for release candidate

Merge pull request docker#1902 from nathanleclaire/plugins

Add Docker Machine driver plugins

Merge pull request docker#1902 from nathanleclaire/plugins

Add Docker Machine driver plugins

bump 0.4.0

Signed-off-by: Evan Hazlett <ejhazlett@gmail.com>