Update compileSdkVersion to 23 (Marshmallow)

[grantland]

Required:

• Add legacy Apache Http Client support:

android {
  useLibrary 'org.apache.http.legacy'
}

• Add ParseSQLiteCursor#setExtras
• Find solution to our org.apache.http usage for android-9 through android-18 (Issue with android M compilesdkversion 23 #231)
• Find a replacement for Notification#setLatestEventInfo(Context, CharSequence, CharSequence, PendingIntent) (Solution Needed)

[rogerhu]

We have a PR open for this one now and just awaiting Maven/Sonatype push credentials.