better error message when not using java8, and updated README, fixes #21
changed way the retrolambda.jar dependency is defined
Fixed typos in README
updated to latest version of retrolambda
added new changes to README
fixed README typo
fixes #22

Find the location of the android-sdk ourselves instead of relying on an
undocumented android gradle plugin property.
updated README
allow plugins to be applied in any order/update retrolambda version
updated README
Update README to make a few things more clear
detect incremental compilation

Conflicts:
	src/main/groovy/me/tatarka/RetrolambdaPluginAndroid.groovy
	src/main/groovy/me/tatarka/RetrolambdaPluginJava.groovy
	src/main/groovy/me/tatarka/RetrolambdaTask.groovy
change java plugin to correctly detect incremental builds