Add support for background-size keywords

Allows the following syntax:

    image '/mr-t.png' cover
Merge pull request #198 from path/background-size-keywords

Add support for background-size keywords
Add vendor support for placeholder
Improved vendor placeholder; added test cases
Merge pull request #199 from devmission/master

Add vendor support for placeholder
add node v0.10 to travis config
Merge pull request #203 from optikfluffel/add-node-versions-to-travis

add 0.10 to travis for testing
Fixed flex display for IE10 to -ms-flexbox
Switched to using 'flex' version in flex method for IE10
Switched to using 'flex' version in flex-flow and flex-direction method for IE10
Fixed alignment for IE10
Fixed flex order for IE10
Fixed 'justify-content' to render 'flex-line-pack' for IE10
Corrected 'justify-content' to 'flex-pack' for IE10
Corrected 'align-content' to 'flex-line-pack' for IE10
Fixed align-self for IE 10

IE 10 uses -ms-flex-item-align to align flex-items. From IE 11 the official align-self property is used.
Update flex.styl
Merge pull request #188 from edgecampus/ie10-flexbox-fix

Corrections for IE10 flexbox
closes #206
expanded contributors section