more flexible signing

if you want to build samples with custom signing keys, add a samples/gradle.properties like this:

storeFile=/path/to/keystore/file
storePassword=xxx
keyAlias=yyy
keyPassword=zzz

if you don't want to build samples with signing keys, simply remove samples/gradle.properties.
delete junk
Merge branch 'dev'
Merge pull request #2 from huahang/master

merge mor flexible signing