github上的很多项目都是用Android Studio开发的，昨天下载一个项目已经好久了，gradle的版本比较低，所以就遇到找不到‘android-library’的错误。从stackoverflow上找到类似的问题，解决方法是用较新的gradle版本。

Instruct Gradle to download Android plugin from Maven Central repository.

You do it by pasting the following code at the beginning of the Gradle build file:

buildscript {
    repositories {
        mavenCentral()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:1.0.+'
    }
}

Replace version string 1.0.+ with the latest version. Released versions of Gradle plugin can be found in official Maven Repository or on MVNRepository artifact search.

com.android.tools.build gradle 1.1.3 all (62) 07-Mar-2015 pom jar javadoc.jar sources.jar

参考：
http://stackoverflow.com/questions/18153739/android-studio-plugin-with-id-android-library-not-found
http://search.maven.org/#search|ga|1|g:%22com.android.tools.build%22%20AND%20a:%22gradle%22