Enable apps to control logging behavior

If an app wants to avoid third-party logging output (say, in production builds)
now the mixpanel Log level is selectable.  Defaults to original behavior.

```
[pforhan ~/dev/pforhan/mixpanel-android (log-control=)]$ gradle clean build
:clean
:compileLint
:copyDebugLint UP-TO-DATE
:mergeDebugProguardFiles UP-TO-DATE
:packageDebugAidl UP-TO-DATE
:preBuild
:preDebugBuild
:checkDebugManifest
:prepareDebugDependencies
:compileDebugAidl
:compileDebugRenderscript
:generateDebugBuildConfig
:mergeDebugAssets
:generateDebugResValues UP-TO-DATE
:generateDebugResources
:packageDebugResources
:processDebugManifest
:processDebugResources
:generateDebugSources
:compileDebugJava
Note: Some input files use or override a deprecated API.
Note: Recompile with -Xlint:deprecation for details.
:processDebugJavaRes UP-TO-DATE
:packageDebugJar
:compileDebugNdk
:packageDebugJniLibs UP-TO-DATE
:packageDebugLocalJar UP-TO-DATE
:packageDebugRenderscript UP-TO-DATE
:bundleDebug
:assembleDebug
:copyReleaseLint UP-TO-DATE
:mergeReleaseProguardFiles UP-TO-DATE
:packageReleaseAidl UP-TO-DATE
:preReleaseBuild
:checkReleaseManifest
:prepareReleaseDependencies
:compileReleaseAidl
:compileReleaseRenderscript
:generateReleaseBuildConfig
:mergeReleaseAssets
:generateReleaseResValues UP-TO-DATE
:generateReleaseResources
:packageReleaseResources
:processReleaseManifest
:processReleaseResources
:generateReleaseSources
:compileReleaseJava
Note: Some input files use or override a deprecated API.
Note: Recompile with -Xlint:deprecation for details.
:processReleaseJavaRes UP-TO-DATE
:packageReleaseJar
:compileReleaseNdk
:packageReleaseJniLibs UP-TO-DATE
:packageReleaseLocalJar UP-TO-DATE
:packageReleaseRenderscript UP-TO-DATE
:bundleRelease
:assembleRelease
:assemble
:lint
Ran lint on variant release: 38 issues found
Ran lint on variant debug: 38 issues found
Wrote HTML report to file:/Users/pforhan/dev/pforhan/mixpanel-android/build/lint-results.html
Wrote XML report to /Users/pforhan/dev/pforhan/mixpanel-android/build/lint-results.xml
:check
:build

BUILD SUCCESSFUL
```
JDK 8 (at least) warns that this may stop being a valid variable name after JDK 8.