Add handling for artifacts with classifiers

[benhylau]

The v0.2.0 plugin is unable to download artifacts with a Maven classifier specified with:

compile 'group:name:version:classifier'

This error is print for that artifact:

"ERROR: Could not find dependency: ${target}"

The correct place to find the file is:
.../modules-2/files-2.1/group/artifact/version/hash/artifact-version-classifier.ext

This change adds support for handling these classifiers. Please let me know if this is the correct fix, and thanks for publishing this plugin.

[coveralls]

Coverage remained the same at 100.0% when pulling 27c7907 on benhylau:classifier into 013e7a6 on ksoichiro:master.

[benhylau]

@ksoichiro ping

[ksoichiro]

@benhylau Thank you for your PR, and sorry for my late reply.

I found that one of the Gradle API, org.gradle.api.artifacts.ResolvedArtifact interface
has getClassifier() method, and maybe we should use this interface to handle artifacts correctly, I think.
But anyway, your fix seems enough to solve this issue.
Thank you!

[benhylau]

Yea I first looked at that interface in ResolvedArtifact, but it's been a while and I can't remember why I couldn't use it. I vaguely remember it had something to do with AndroidDependency being a custom class. Thanks for merging!