Email components hierarchy created.
SwiftMailer applied as email solution.
Unit tests for \yii\email\swift have been improved.
Methods 'addText' and 'addHtml' added to \yii\email\BaseMessage.
'\yii\email\ViewResolver' added.
Email message render functionality added.
Email message interface extracted.
Merge branch 'master' of github.com:yiisoft/yii2 into email-swift-2
'email' renamed to 'mail'.
'SwiftMailer' recomposed into extension.
Vendor testing support provided.
'swiftmailer' extension unit tests fixed.
'MessageInterface::createAttachment()' renamed to 'MessageInterface::attachContentAsFile()'
Method 'MessageInterface::__toString()' added
BaseMailer::defaultMessageConfig published as BaseMailer::messageConfig
Merge branch 'master' of github.com:yiisoft/yii2 into email-swift-2
Merge branch 'master' of github.com:yiisoft/yii2 into email-swift-2
Interface 'MailerInterface' extracted.
Method 'MailerInterface::createMessage()' added.
'yii\mail\ViewResolve' removed.
Interface 'ViewContextInterface' applied to BaseMailer.
'yii\mail\MessageInterface' file related methods interface adjusted to use options, embed file methods added.
skip expiry test for memcache on travis

fixes #877
print travis ENV
travis memcache env check: try again
simplified travis env check for memcache(d)
'yii\mail\MessageInterface' methods 'addText()' and 'addHtml()' removed.